So for those who didn't understand why the Lethe gave Nico's memories back, that's the artifact's power. If it is in contact with both a person and a liquid, and the person is touching the liquid, then the liquid will have the opposite effect on the person. So the Lethe water was reversed, and gave Nico's memories back. Characters will be OOC. Sorry for not saying this earlier, but this takes place after HOO, with some changes, and will use plenty of things from the AOT manga, though there will also be contradiction. Undertale AU's may make some appearances, as will some theories.

As soon as Nico heard the voices, he passed out. As he lay there still, his mind was brought to a rather spartan room. In two chairs, sat two very different people. Nico sat down in a third chair, leaned forward, and asked the obvious question.

"Why the fuck are you in my head?"

The one on the left, which looked like a skeleton, yet anatomically incorrect, and Nico couldn't feel him with his powers replied first, and with a laugh,

"That's what I said, too. We are your past lives. We can look through your memories, you have access to ours and our powers, and if you are unconscious, or give us permission, we can control your body. This is one of the biggest reasons that children of Hades go insane, they are the only ones who can hear us. Thankfully, you were given the memories before you grew too old to accept them."

Nico looked warily at him, "So you can control me? I don't really like that idea."

"No worries," the skeleton shrugged, "I'm pretty done with life, anyways. By the way, to reintroduce ourselves, I'm Sans, and this here is Levi."

Levi looked at him with black eyes, so like my own, filled with repressed grief for the dead, and jaded from seeing so many of said deaths.

"Hey kid, nice job with freeing us, but how did you do it?" he asked.

"First off, a magic artifact, and an accident. Second, who are you to call me kid? You're as tall as I am!" Nico replied heatedly.

"That may be true, but I am older than you, as i am 34. And no, Lotus Hotel does not count." Levi snarked back.

That stopped Nico in his tracks. "You can read my mind?!"

Levi sighed, "This is your mind. We live here. Of course we can read your mind."

"Alright, alright, cool down everyone," Sans said, "Same side, k? So now, Levi, should i go first or should you?"

"I'll go first, as it should seem more familiar."

"Cool. So, kid, Levi is gonna give you his memories and abilities, all right?"

Nico nodded hesitantly. "Will this change my personality at all?"

"Possibly, but not directly, just from your reaction to this," Sans replied, gesturing at himself and Levi. "Our personalities won't become a part of yours."

Nico nodded again, more confidently this time, and all of a sudden, memories flowed through him. He managed to remain on his chair, and recovered rather quickly.

"Right, so now it's my turn. In this area, you cannot use our powers or abilities, and so you will feel stronger only when you wake up." Sans rubbed his hands together.

Again with no warning, the memories flooded his head, this time far more, several times as many.

"And now, you will awaken. We can still talk to you, and you may gain some of our features. We'll find out which later." Levi said tiredly. "I'll be sleeping for a while though, so you won't hear me talk at all for about an hour."

Nico bolted straight upright, smacking his face into the not so soft face of Percy Jackson. The ever so great Hero of Olympus, fell straight on his butt, groaning in pain. This caused laughter all around them, as Nico looked around, seeing the entirety of the Seven, as well as Reyna, seated around his bed, which he then realized was in the infirmary. Clearly, someone had found him.

"Clearly," Sans remarked.

"You shut up now, k?" He thought.

"Ok. Have fun explaining to your friends why you passed out," He snickered.

Nico gave him the mental equivalent of the middle finger.

Percy sat back up, with a sigh at his friends, before looking at Nico with concern, "You ok, Death Breath? We heard a crash, and then we found you lying on the ground, knocked out."

"I'm fine, just some memories returning. Guess that this artifact really is dangerous. I remember my childhood now," Nico said with a smile. He remarked to Sans, "I thought it would be best to not mention you two."

"Probably," he agreed.

"That's great!" Hazel exclaimed, hugging her brother. Frank smiled at him from behind Hazel's head, showing his appreciation that Nico was alright. Nico smiled, hugging Haze back, before Percy tried to clear his throat, failing badly, and then started coughing. Annabeth laughed at him, before hitting him in the back loudly, stopping him.

"So Nico, the Hunters are back in Camp, and we are having one last CTF game before heading to Olympus for the Giant War gifts." Percy rolled his eyes. The gods had declared that the heroes would have to wait until after some construction had happened, and the throne room was fixed from the battle damage. Aeolus, controlled by Gaea, had attacked Olympus, but the combined force of Nemesis and Hecate had successfully defended Olympus, though not without damage to it.

"That definitely didn't take forever," Nico said sarcastically. "The gods don't want to present us our gifts in a destroyed room, Nico," Hazel admonished. "Be at least somewhat respectful."

"Sorry, sorry," Nico held up his hands in surrender. "I'll try."

Hazel smirked at him, knowing that she had won.

"So are we gonna play or not?" Frank broke in.

They laughed and jogged towards the forest, chattering about random things. When they arrived, Nico was told that he was on defense.

Defense.

One of the more boring jobs normally, but it was a necessary position, and you got plenty of action against the Hunt. The campers were admittedly awful, particularly the Olympian cabins, with the exception of Ares, Hermes, and Hephaestus. Apollo was decent, but their skillset was more based off of being able to trade out wounds extremely well due to their healing. As such, the Hunters kind of just ran right past them.

Nico was grouped up with the Hecate and Hephaestus campers, who decided to create a magical field. They used their knowledge of magic and metalworking to create a trench and wall all around the flag, followed by a magic barrier that prevented their being any silver in the area.

This meant that the Hunt would be almost completely unable to enter the area. Nico the summoned a good dozen skeletons to stand guard by the walls, as they couldn't really be hurt by arrows. Nico had also figured out that his geokinesis could be used on the Hunt's silver itself, meaning he could just fling everybody around.

As the horn blew for the start, Nico, Levi and Sans all said the same thing.

"Let the battle begin."

So I hope you enjoyed what you read. Frisk and Chara will make appearances, and they will both be female. Undertale is both real and a game, so there will be people who recognize the monsters, and AOT is the same, with the location being on a remote island. Titans are currently extinct, but will make a reappearance.

Please review with any ideas on where the story should be going.