Author's Notes (There are several):

*This chapter was difficult to write because it didn't seem like Cas got a lot of info about what was going on on the outside while Lucifer was possessing him. He seemed a bit clueless trapped in his own mind watching that stupid TV. I did the best I could, let me know what you think.

*TRIGGER WARNING for round-about, not-graphic mentions of rape. It's not really something I focus on in the story, or even really this chapter, AT ALL, but including it is necessary for content and so I had to include the trigger warning.

*Again, there's more information about the Demon Blood Bond from Ember's point of view and that of others in the sister story, Ember: Half-Demon, Half-Human (though it's no more or less graphic).

*I do not own Supernatural, unfortunately, or I'd be making more money.

Christmas

Things had gone slowly.

It hadn't been the worst Christmas ever. That had been the Christmas he'd spent in Purgatory. Dean had reminded him that it was Christmas, and they'd celebrated it (if you could call it that) by killing an extra three vampires and two werewolves.

Castiel had never celebrated Christmas in what Sam, Dean, and Ember considered the "normal" way until he met them. He had celebrated when Christ was born, of course, but it hadn't been on December 25th. Castiel's first "Christmas" had been the year of the Apocalypse, and he had decided that there was something pleasant in the small traditions that Bobby Singer and the Winchesters continued year after year… something almost magical.

After that had been the two-year civil war with Raphael. The next time Castiel had celebrated Christmas had been with Daphne, when he had lost his memory. They had had a pleasant, if boring Christmas during which they had fed the homeless and talked about their mutually beneficial relationship. It had been a loveless marriage, but he protected her from her ex-boyfriend, and she protected him from the government who had been pestering him for citizenship papers. Since he had reclaimed his identity as Castiel, he had learned how to forge citizenship paperwork from Sam if he needed to. He had gone back and smited Daphne's ex-boyfriend, who was a demon anyway.

The year after that had been the worst Christmas ever, in Purgatory. Afterward, in 2013, he had returned to the pattern of spending Christmas with the Winchesters and Ember. He had recently once again become an angel, and being in a relationship with Ember had been almost wonderful enough to allow him to forget Dean's absence.

The previous year, 2014, Castiel had spent Christmas on his own in order to avoid Ember.

This year was a bit like the year of the Apocalypse, or the one after Purgatory. Christmas was fun, and festive, and spent with friends, but he never really liked seeing Ember and Dean together. There was something different, though: he could swear that Ember shot him several sideways glances when she thought Dean wasn't looking. She didn't put her hands on Dean as much as she always had in the past, either.

He wasn't sure what it meant. He liked it, but perhaps she was just being courteous because they used to date. He wished she wouldn't – he didn't need her pity, on top of everything else.

"Cas," Sam said to Castiel on Christmas night.

"Yes, Sam."

"Look, there's something I wanted to run by you," he said.

"Sam, tell me you're not talking to Cas about those visions of the cage you keep having," said Dean, striding up behind them.

Sam looked ashamed. "They're happening more and more frequently now," Sam said. "I just want to know what they mean."

"Doesn't matter," said Dean. "The answer is still no."

Castiel looked at Sam in question.

"I've prayed to God a few times since we let out the Darkness," Sam confessed. "And each time, I see the cage… each time I have a… a vision, I guess you would call it. And it's always the same."

"He thinks it means he needs to go to the cage for help," said Dean.

"Sam, no," said Castiel immediately.

"But didn't Lucifer help seal up the Darkness the first time?" asked Sam.

"Yes, but that was a very long time ago. Sam… there must be another way," Castiel insisted.

"That's what I told him!" said Dean.

"Hi, bitches!" said Charlie, coming up behind them suddenly. "Guess what I got you for Christmas?"

And the conversation was over.

January 18

Castiel knew the angels were preparing for a mass smiting toward Amara; he could hear it on angel radio. She had been active, lately, targeting and killing churches and pastors – probably trying to get God's attention, he theorized. He couldn't participate because he was not welcome back in Heaven, which is where the smiting would originate from. Still, he drove as close as he safely could to the point of smiting, just in case he was needed.

He was glad he had done this. When the smiting was complete, he found Dean just outside of the smiting radius, throwing up. "You're suffering from smiting sickness," he explained to his friend.

"That... that's a thing?" Dean asked incredulously.

"Yes," Castiel said. "The angels... what they did... it released a tremendous amount of energy and there's fallout, so this whole area is poisoned."

"You can heal me right?"

"No, I can't," Castiel confessed. "And the closer you get to the blast site the worse your sickness will become."

So Castiel went to investigate the smiting alone, happy to have stopped his friend from continuing. It would've meant an untimely and painful death for Dean after about 10 minutes.

The trees were eerily silent, and it was too dark to be daytime. Castiel finally found Amara, the Darkness, towering over another angel. The other angel, Ambriel, fell to the ground dead as Castiel watched. Amara looked skyward and seemed to inhale the darkness that had been plaguing the sky - it was daylight once again.

Finally Amara turned toward him. "Castiel?"

"Amara, what did you do to her?"

"I consumed her," responded the woman. "Her grace, her spark, her memories, everything... she... oh, she really hit the spot."

Castiel took out his angel blade.

"Seriously?" Amara asked him sarcastically. "I mean Heaven brought the thunder and it barely even scratched my paint job. But you and a shiny knife, sure, that's gonna work."

"You think I'm afraid to die?" he challenged.

"I know you are," she responded. "You reek of fear and self loathing."

Castiel raised the blade to strike, but Amara grabbed his arm. "Oh, scary."

She flung him against a tree, and he felt the sharp stab of pain. "Why God took a special interest in you, I'll never understand. My brother always did have horrible taste in men."

"Just do it," Castiel snapped at her.

But Amara brushed her hand against Castiel's cheek. He shuddered from the contact.

"Blue eyes, you're not even worth the effort... and no offence, but you look a bit used up. Plus, I have a job for you."

He felt pain on his chest – a message, he understood, written on him in blood. Then he was being transported away…

~ANGELS WILL FALL~

Castiel ended up in Crowley's throne room at the entrance to Hell. He wasn't sure if it was because the message Amara had written on his stomach was meant for Crowley, or because Amara knew of his friendship with Dean and had sent him to Dean's side. Either way, he ended up in a room with Castiel, Crowley, Dean, and Rowena.

"Amara," he explained to the group. "She... she's alive. She... she sent this message." Castiel opened his trench-coat and shirt to reveal a message carved into his chest.

"I am coming," Crowley read aloud. "Is that a threat?"

"Or a promise?" asked Dean.

Suddenly, they heard screams.

It was Sam, Castiel could tell… and Ember.

No. Surely the Winchesters and Ember hadn't attempted to speak to Lucifer! Surely that wasn't why everyone was gathered here, at the entrance to Hell…

But Castiel knew from the large pit of dread in his middle that that was exactly what had happened.

Something had gone wrong, of course. The witch, probably. The witch had betrayed them. Or possibly Crowley… Either way, Sam and Ember were now trapped in the cage… with Lucifer. He and Dean ignored Crowley's warnings and tore down the hallway toward the sound of their screams.

"Hey, ass-butt!" Castiel yelled, the words wrenched from his throat once again in a time of need. He could see the cage, not in the depths of Hell like it should be, but somehow magically transported here. In the cage was Lucifer, standing over Sam, who was bleeding onto the floor. Ember was also sprawled across the floor of the cage, unconscious. Castiel fought down fear and panic as he looked at her, noting that the spark of life had not yet left her.

"Dean," said Lucifer when Castiel and Dean stopped just in front of the cage. "Ah, the other one."

Lucifer snapped his fingers and they were magically transported into the cell with him. "Welcome to the party."

"Scared?" Lucifer taunted.

"Not even a little," answered Dean.

Castiel got out his angel blade, preparing to fight.

"Ah, ah, ah," said Lucifer. "At moments like this, it's all about ambiance." Lucifer snapped his fingers and music started playing - Heaven Must Be Missing an Angel. He started to dance, mocking them.

Castiel lunged for Lucifer, and Lucifer grabbed him while Dean attended to Sam and Ember. "I can beat the Darkness. Me. Only me," he said. "Who are these two? They're a couple of apes. You know I'm right, Castiel. You know it!"

Castiel struggled away from Lucifer. "You wanna play that way?" Lucifer taunted, and punched him repeatedly in the face.

Castiel went down next to Ember, and though his own nose was bleeding, healed her quickly. "Help us!" Castiel screamed, launching himself at Lucifer once again.

Castiel saw Dean fly backward away from Lucifer, landing almost on top of Ember. Ember flung herself up and hurled as much force power as she could at Lucifer, but he only laughed. "That doesn't work on me… it tickled, though. You'll have to go farther toward your demon side than that."

Lucifer kicked Sam in the stomach, and then grabbed Dean around the neck and slammed him against the bars of the cell. Dean grimaced as Lucifer lifted him off the ground, choking. He looked at Sam. "Alright Sam," he threatened. "I'm gonna make this real easy for you. You say the magic word or your brother dies... and we both know you won't let that happen."

Castiel charged at Lucifer again, allowing Dean to drop to the floor. He felt Lucifer's repeated punches to his stomach, and he fell to the ground. Lucifer stood over him, foot on Castiel's angel blade, red eyes staring into blue.

"So, last words," said his older brother.

"Can you really beat her?" Castiel asked him telepathically.

"I can," Lucifer answered.

"I have conditions," Castiel said, "Which must be agreed to, vel inanus pactum et reditus."

Lucifer's eyes flashed a dangerous red. If Castiel agreed to be a vessel and Lucifer didn't abide by Castiel's conditions, he would be immediately cast out and returned to the cage. It was not the choice of vessel Lucifer wanted, Castiel was sure… but he would take it, to escape the cage.

"Name them," he said brusquely.

"You cannot hurt or kill the Winchesters or Ember," Castiel said, "Or I will immediately return to control. You must fight Amara as soon as you have the means and the power, and any lack of effort will result in your immediate return to the cage. And, after you defeat Amara, you will return to the cage, and we will discuss your fate further."

Lucifer frowned, narrowing his eyes at Castiel. The conditions were far too constrictive for him, Castiel knew… but he would take anything before he would be left down here in the cage.

"Ego faciam estud," Lucifer said dryly, agreeing to the conditions.

"Then yes," said Castiel.

February 17

ONE MONTH LATER

Lucifer didn't allow Castiel to see what was going on. This annoyed him, but he supposed he should've expected it. It didn't matter – the conditions with which Castiel had accepted Lucifer as his vessel would hold true.

Castiel could sense, at least to some degree, the amount of power Lucifer was using at any given time – whether he was flying, or fighting, or killing. Castiel attempted to see what was going on on the one occasion that Lucifer made a kill, but his older brother became angry and responded with pain. It didn't matter, Castiel told himself again – Lucifer was bonded by his conditions. He would try to test them, Castiel knew this… but he would know when this occurred.

Then he smelled Ember. Her scent wafted to him even in the recesses of his own mind, and he fought for control. He felt Lucifer's slice of pain but ignored it, clawing his way to the surface.

The shock of seeing Ember's face pushed Lucifer away, and the sight of her black eyes banished Lucifer to the recesses of his mind. She was a demon? How?

"Ember!" he exclaimed. He scrambled backward from her, alarmed.

"Castiel," she said, smiling, though the smile didn't meet her eyes. "I was wondering when I'd be seeing you."

"What's happened!" he asked. "What has he done?"

"Demon Blood Bond," she said, shrugging.

"No!" he said, horrified. She had too much skin showing, and Castiel fought with the bed covers to cover her, panic rising in his throat. "That's impossible! Lucifer-…"

"Is the father of demons," Ember reminded him. "He can do whatever he wants." She smirked at him.

"No!" he screamed, and he reached into his pants pocket for his phone… but his phone wasn't there, and neither were his pants. A slice of pain shot across his entire body, and he knew Lucifer was attempting to regain control. He did the only thing he could think of… he ran for the door.

She teleported, standing in front of him, and dropped the blanket. "Stop holding back! You did this to me, again, at least enjoy it! We can be together again… sort of…"

Another white-hot flash of pain, and Castiel could feel himself losing the fight against Lucifer.

~ANGELS WILL FALL~

Castiel ran it through his mind over and over again. He obsessed over it. There was nothing else to do, here, trapped in his own mind.

How could Lucifer have forged a Demon Blood Bond? Clearly the Blood Bond had been forged between Ember Castiel, of whom Lucifer currently had control. Even now, he could feel the bond, in his mind, a small but pulsating tether tying him to her. But how was that possible? He and Ember had slept together, yes, and Ember was half-demon. But he was an angel...

More importantly, what had gone wrong with the conditions he had set forth for Lucifer? His only thought was that perhaps he should have clarified the definition of the word "hurt"… "You cannot hurt or kill the Winchesters or Ember." That had been the agreement. But clearly Lucifer had found a loophole.

February 18

Just when Castiel was most miserable, Lucifer was suddenly banished to the furthest recesses of his mind. Castiel knew immediately what had happened – he had broken the conditions, in some form or other. Sure enough, Sam was lying in front of him on the floor, unconscious.

Castiel laid a hand on Sam's head. While he healed his friend, he read his mind.

Sam and Dean had embarked on a journey to get the Hand of God, a supernatural relic said to wield great power. It had been sunk on an American submarine during WWII, so Lucifer, pretending to be Castiel, had sent Dean back in time to retrieve it. Lucifer was supposed to accompany Dean, but he had been kicked off the submarine almost immediately by angel warding. They had been looking for a way back onto the submarine so that Dean could get home. Sam had found a spell to dispel angel warding called the "Spell of Gathering," but it needed the power of an archangel.

"Well Sam, we may as well try," said Lucifer.

"Are you nuts?" Sam said, alarmed. "You're not strong enough, Cas! You could get hurt!"

"You find a better option?"

"Well, no," Sam admitted. "But… without a serious boost to your angel power, that spell won't even work!"

"My strength may surprise you," said Lucifer.

"Wait a second," said Sam. "I remember Bobby told me when you needed strength to retrieve us from the past, you used him to power up. You touched his soul, right?"

"That's right, I did that," said Lucifer, still cutting ingredients. "But that procedure can be fatal."

"Use my soul," Sam said earnestly. "That way maybe you'll have enough power to wield the spell."

"That isn't necessary," said Lucifer.

"It's worth the risk," Sam argued. "Cas, Dean needs our help. I trust you."

Suddenly, Lucifer began to laugh, dropping the façade of Castiel. "What?" Sam asked, confused.

"Oh, it's just - I don't need you anymore," said Lucifer, smiling. "I mean, Dean's the one with the link to Amara. Why have I been trying to spare you? I mean, maybe it's because you're like the girl who kept turning me down at the prom."

Lucifer slammed Sam against the wall. "I will touch your soul," he said in his high, creepy voice. "Just because you asked so nicely. And I'll use your spell to blast through the warding and retrieve Dean and the uh, Hand of God, and then when Dean comes back and he finds this place decorated with your guts, I will tell him the truth, Sam. I'll just say, 'Dean -…"

At this point, Lucifer switched back to his imitation of Castiel's deep voice. "… Dean. He knew the risks. He wouldn't take no for an answer.'"

"Lucifer," Sam said in recognition.

"In the flesh." Sam watched, horrified, as Lucifer put his hand inside Sam's gut to feel his soul. And then Sam lost consciousness.

In the few seconds it took for Castiel to realize what had happened, he could already feel Lucifer scrabbling for control again. It was a dull thud in his head, nothing near the usual white-hot pain. Lucifer wasn't strong enough yet to take control back, but he would be, soon.

Sam was awakening. He took one look at Castiel and scrambled backward.

"Sam, it's me," said Castiel.

"Cas?" Sam asked. "Why?"

"I wanted to be of service to the fight," Castiel explained, understanding Sam's question. The dull thud of pain was getting louder. "And only Lucifer can beat her."

"You chose this?" Sam said in horror. "You have to fight, Cas! Eject him now!"

"I can't!" Castiel said honestly. "It's taking all my strength to keep him from killing you. And besides, we need him!"

"No Cas, we don't!" said Sam. "We'll find another way to stop Amara!"

Castiel shook his head. "We need him to save Dean."

"You can't time travel," said Sam.

Castiel shook his head again. "Only Lucifer can. Listen. And this is very important. Ember-…" He felt the white-hot pain. Lucifer would regain control soon. "The Demon Bond. Again."

Sam looked at him in horror. "But how? Lucifer's not-…"

"Archangel," Castiel said, and the pain consumed him. "Made… demons. Can do whatever-…" More pain. "You need my blood, to undo it," he finished. He pulled a flask out of his pocket, his hand shaking, and cut his hand, his blood dripping into the flask. He gave the flask to Sam, who pocketed it. Then he gave himself over once more to Lucifer.

Early April

SIX WEEKS LATER

Days turned into weeks. Weeks turned into months.

To cure Ember of the Demon Blood Bond, the Winchesters would need to combine Castiel's blood and Ember's blood. Then they would need to track down Lucifer one more time, find a way to trap him, and say the incantation while pouring the blood over him or making him ingest it.

Perhaps the Winchesters had not been able to get Ember's blood, or the vial of Castiel's blood had been destroyed somehow. More likely, they had not been able to track down Lucifer for a second time. Either way, Castiel knew that Ember was still a demon.

He could smell her sometimes, and when he smelled the familiar mix of oranges and demon and something ethereal, he would fight for control. Sometimes he won. Sometimes he didn't. There was always pain. The few times that he was able to regain control, he was horrified at the things he saw.

He was miserable. She was a demon – again - and it was all his fault. She would never forgive him for this, and he would never want her to. Castiel lost track of the amount of times he fought for control and won, only to find himself with her in ways that, in another situation, would have been a happy dream.

Castiel didn't know much about how the Blood Bond worked, but he knew a little. He knew it had been used as a conversion method by demons for thousands of years. If a demon seduced a human while feeding them demon blood, it would invoke the Blood Bond. It took years to torture a soul enough to demonize it, while armies could be created much more quickly in this way if needed: When two souls have been bonded, if both with demon blood, the demon blood shall rule.

Castiel also knew that the Blood Bond boosted the demonic powers of the victim to a level equal to that of the pursuant, with some variation depending on how much demon blood the victim had ingested at any given time. With Dean, Castiel knew that Ember and Dean had been equal in power. But Ember's Demon Blood Bond now was with him, so Ember's power would be equal to his, but less than Lucifer's.

How many times since Lucifer had invoked the Demon Blood Bond had he forced Ember to kill or hurt someone for him? How many times had Ember been tortured when her human side won out and she had refused? Once when Castiel wrestled control away from Lucifer for a few moments, he saw the marks of a whip on her. She grinned at him. "Look what you did!" she taunted him maniacally. "Well, not you, I suppose… another demon, on Lucifer's orders."

"I'm sorry," he whispered.

"I don't care," she answered, her black eyes flashing. He had the impression that she knew exactly how much her words hurt him, and that his misery pleased her demon side. "At least I get to have a little fun," she said, smirking.

Castiel lost track of time, though he thought perhaps it was early April. Then, suddenly, finally, he felt Lucifer's presence relinquish.

Castiel was surrounded in a ring of holy fire, staring at Sam and Dean Winchester.

"Cas," Dean said quickly. "Before anything else, it's Ember."

There was a flash of white-hot pain. The Winchesters had used a spell, but Lucifer was far too powerful for something like that. He shook his head. "I gave my blood. She -…"

"Lucifer destroyed it."

Castiel wasted no time. Shaking, he cut his arm, dripping blood into Dean's waiting flask. The moment the blood was in the flask, however, there was another white-hot flash of pain, and he lost control once again.

He sank back into his own mind, feeling for the small tether that was his Demon Blood Bond. It was gone! Somewhere, Ember was herself again. Somewhere, he knew, she would be unequivocally distraught, and overwhelmingly guilty, and alone.

She would never forgive him, just as he would never forgive himself. Dean would help her, though, as he had last time. At least she was free. And it was a relief, because he didn't think he could keep fighting Lucifer for control - he could still feel the residual pain from the last several times he had tried.

Now, it was up to Lucifer.

April 7

ONE DAY LATER

Something was different. Lucifer was gone. He was still with Castiel, in his mind… but he was unconscious.

What had happened during Lucifer's run-in with Sam and Dean?

Castiel took control easily, and immediately understood why Lucifer was unconscious. The pain… it was unbearable. He had never felt pain like this before. Were it not for Lucifer's power still residing inside him, he would be dead. What had happened to him?

Amara. He could see her across the room, facing away from him, her head cocked to the side in thought.

He tried not to allow the pain to pull him under. It was overwhelming, and he could feel it in every atom of his being. It was worse than when he had been tortured by Malachi's man, far worse even than his torture in Heaven…

"You're not the angel I was torturing," Amara said. "Where is Lucifer?"

But Castiel felt his eyes close, and he knew no more.