Castiel had been a silent observer well before time was a documented construct.
He had long been ordered to watch, but not interfere. At this moment, especially after his recent "dust up" in Heaven, as Dean might say, he feels it to be particularly necessary to stay out of the sight of humanity. He would only partake to deliver his instructions to his charge, as he had been told to.
Standing in the field of dead machines, he could still hear Sam screaming in the basement where he had been locked to purify himself of demon blood. He had no desire to get closer, even though he had been ordered to do so. Still, something briefly held him back here. As the seals were broken across this plain, he could feel them, and he heard over the angel radio. Things were time sensitive, so he had to answer Dean's prayers, he had to make Dean swear to the service of his Father, and then break Sam out of the panic room. Yet he still held back for a moment more, perhaps faintly aware the next steps he took could very well rend the brothers permanently.
"So far from home, caeles." A voice dripped from the darkest part of the shadows. It slid from where the light could not reach, a place not many chose to exist in.
Castiel did not fear this newcomer, but he certainly had not been expecting them.
"Yet you are right in your element, are you not, mahr?" It would be impolite to ignore such a demon. Historically the two species were not amenable to one other, and Castiel knew other angels in his very own garrison that would not have been so well-mannered. But at this moment he would not risk making an enemy, especially not one so old and powerful.
"Young, yet not without knowledge." Then suddenly she was beside him, in a garb he knew was not made in this realm, or this age. He turned to see her, and even being celestial, had trouble keeping his gaze steady. He was having just as much trouble fathoming how much older she could be as he did taking her presence in.
Draped in what he could only describe as darkness continuously gathering and reshaping, the mahr had taken on a human guise, yet upon a closer glance, it was as if a statue of a Greek myth had taken on life. Slightly too pale, slightly too perfect, blessed with eternal youth, yet centuries, if not millennia, old. According to legend these were also hideous creatures, ones that ruined and twisted everything they touched. He could only remember a handful of times he had crossed paths with these demons, and this one resembled a demon he had seen in a Germanic town in one of the later centuries. As such he carefully used the title 'mahr', as old demons did not like their origins being confused.
"What brings you here?" He could not be too careful at this juncture – mahrs were fiercely independent demons, and reported to no one in Hell. Castiel supposed their loyalty lay only to Lucifer, but even then, their purpose only served themselves. He shuddered to think that she was here with any purpose. "I must warn you, any ill intent towards the Winchesters or Singer, and the wrath of Heaven will find you."
After a second of silence, she threw her head back and laughed a moment, the sound like dry pines snapping as a fierce wind piercingly whistled past. Castiel found he disliked the noise immensely; it was nothing like Dean's bark of amusement or Sam's quiet chuckle.
"These dreamers have nothing to fear from me." Pulling herself up to her full height, her arms clasped each other under her breasts and Castiel thought for a moment she appeared almost regal. "How can any mahr ignore what young Samuel Winchester has created for himself?"
"You…came to observe?" Castiel was intrigued he was not alone in this right.
"They are hallucinations, yet his own mind has merged them into a nightmare he cannot untangle himself from. It is rare for us to find a human who has built a worse dreamscape than what we can whisper to them as they sleep." She had not stepped any closer, but stretched her neck, almost properly mimicking the motion of a human craning their neck to better watch and listen.
"Whisper?" Castiel scoffed, he had seen mahrs at work. "You sit atop them as they sleep, trapping them in physical form so they are helpless to your machinations." His time spent with the Winchesters had made him defensive of humans, and he realized his words had been impulsive. Luckily, she did not take the bait.
"Even burdened and distracted by whatever task assigned, you must realize he is fragile. Too much more of this and he will be useless." Turning to face the angel caught off guard, the smile that played across her lips was genuine. "What, you think an unattached demon doesn't hear prophecy? Does not read? Lilith holds no power for us, but she is nonetheless interesting to watch."
"Take your leave, mahr, you've long outstayed your welcome." Castiel ground out, becoming aware his vessel's hands were fisted with anger. How she knew of the roles that Heaven had planned for the Winchesters he was not privy to, but it irritated him greatly.
She turned, her long garment twisting about her, and magnetically pulling at the darkness on the ground in towards her. They were nearly shoulder-to-shoulder now, but he continued to look onto the house, whereas she faced the night behind him. The momentary silence was interrupted by Dean yelling for Castiel yet again, which was cut short by an agonized howl that had torn its way from what remained of Sam.
"Perhaps here I have." She seemed indifferent to his feelings on the matter. "Remember, caeles," Castiel scowled at the Latin moniker bestowed upon him, "every creature can have nightmares."
Turning to remind her of Heaven's power, he found her to be gone. He searched the surrounding darkness for the mahr's presence and detected nothing. As he began to silently move to approach Dean, her words bothered him like an uneven flight feather in his wings. She was correct, every creature had something to fear, so every creature had the potential for a nightmare. He flew the rest of the way, standing in the garage, semi-cloaked in shadow, and watched Dean pace and yell. Momentarily forgetting his orders, he contemplated telling his charge of the watchful demon who had come to watch Sam as if he were the subject of a surgery theater. To do so would enrage both Dean and his superiors in Heaven, so he decided against it, but seconds before the older Winchester caught sight of him, Castiel glanced into the darkness to make sure the mahr had truly taken her leave.
For a moment he thought he saw the unnaturally-shifting darkness, but Dean had seen him. The thought of the demon was vanquished to the back of his mind, and he moved to appeal to what he hoped was his charge's protective half. He knew he was manipulating him, which pained him a great deal, and as he listened to Dean swear his service, he wondered if a mahr perched atop a sleeping victim had purer intent. He hopefully prayed this was untrue.