A/N: Behold, the first (and probably only) story featuring one of the most (if not the most) crackiest pairings in existence! However, despite the pairing being crack (did I mention it's a crossover as well?), I'm going to do my best to make it seem as believable as possible. I hope I succeed with that.

Warnings: These will change for each chapter. The prologue has no warnings.

Pairing: Vergil/Sagi.

Theme prompt: Obsession.

Try to enjoy the crack my brain has produced.

Demonic Humanity
Prologue: The demonic aura

It was an unhealthy obsession, and an unnatural one. There were plenty of people that Vergil would see in the run of a day, dying on the street, and he would pay them as much attention as he would the air. Death was a natural part of life, and the strong were the ones who survived. He clung to this logic for years, ever since the death of Eva. It seemed to do him well, until that damned night where he found a boy dying on the street from a beating. Vergil had grown accustomed to seeing this, so he paid him no mind. This was until he sensed the demonic aura pulsing from the boy. The aura was nothing he had ever felt before. It was a strong power, and it screamed at him. It screamed that it could rival that of his father, Sparda.

It was because of this aura that the half demon took him in, and that was the sole reason. It was not an "act of kindness", as mortals would put it. It was an act of nurturing a power that rivalled Sparda's. Nothing else. He took time to hunt demons so he could get particular potent healing items, items that would speed the healing process. Vergil needed this boy to heal as soon as possible, so he could ask him what kind of power he possessed. Each day that passed, the aura glowed brighter, and its raw power pulsed in the room. The feeling of that power coursing through his veins made Vergil shudder in anticipation. This boy would be useful to his plans of retrieving ultimate power that he may not have to raise the demonic tower, Temen-Ni-Gru. That, however, depended on how soon the boy woke up from his unconscious state. The half demon was patient, but even he was impatient in certain circumstances.

The abandoned library near his residence was his favorite place to visit, as it hosted various books on his father, along with how to raise hell (to put it in simple terms). It had been abandoned because demons tended to frequent it for a feeding ground, yet Vergil had quickly disposed of the hell prides haunting the library. He needed no disturbances when it came to studying about Sparda and rituals demonic in nature.

His ice blue eyes scanned through the pages worn down by the years, his fingers turning each page with such a gentleness that it would not leave the slightest mark. These books were precious, and they were not to be disturbed with scratches or tears. As he snapped shut the book when he was finished reading, his eyes averted to the figure on the couch. The boy was clutching onto a pillow, muttering incoherently with a pained expression on his face. Vergil mused that it was because he was remembering the beatings that those people had given him that night, but he paid it no mind. Humans had to deal with their own issues. One boy who had a strong demonic aura was not going to make Vergil play mother to him. The thought disgusted him.

A clicking sound echoed through the halls, which in turn caused Vergil to avert his eyes from the book he had begun to read and look up. They set themselves on the boy for mere moments before staring straight ahead. If this person came for the boy, they would have a fight on their hands. Vergil was not willing to give up such a strong tool without a bloodbath.

The man that appeared before him was a human, the half demon knew as much, but he also knew that he was trying to become a demon. The moving, cancerous looking redness on the right side of his face was enough of a giveaway for Vergil without the annoying aura he was emitting. It was neither human nor demon. It was not the aura of a half demon, either. It was the aura of someone stuck between human and demon, yet not half demon.

He stopped a few feet in front of Vergil, his eyes looking around the room before setting themselves on the half demon. Vergil was not one to be shaken, yet the stare the man was giving him was enough to make him question who he was, and what he wanted. "So, you're looking for the book of ancient legends," he began, eyeing the book in half demon's hand, "the tale of the demon warrior, Sparda?"

Annoyance crept through Vergil as he snapped the book shut. "That's not what I'm looking for," he placed the book back on the bookshelf, his eyes narrowing, "leave me."

The man stepped out into the light, which made the half demon able to see his features more clearly. The cancerous mark on his face was moving, and his aura was causing the annoyance to build inside of him. He could place a person's aura instantly, but this man's aura was mixed up to the point that he was unable to tell what he was. "Then what ARE you looking for? A demon who impregnates a woman, who then bears twin sons... That's the story, isn't it?"

In one swift motion, the half demon took out his prized katana, Yamato, and pointed it at the man's neck. "Leave me. I won't tell you a third time."

"People inherently fear evil... however," the man began to say, placing his hand on Yamato's blade, "occasionally, people may become seduced by evil." Blood began to drip onto the floor from the cut that Yamato was giving the thing's hand, yet he seemed to pay it no mind.

At this point, Vergil was becoming confused. He sheathed his katana and narrowed his eyes at the man. "What are you getting at?"

"Share with me, the story of Sparda..."

That was the last straw. Vergil wished he could take out his katana and stab the man, but he knew that he would prove useful to him, just as the boy on the couch would. However, the boy would serve to be more useful than the man, for his aura was a pure, powerful demonic aura. This man's aura was a jumbled mess. He was weak but strong, useless but useful. It sent the half demon so many mixed signals that he was giving himself a headache thinking about it.

Vergil began to walk away from the man, but not before he noticed those disturbing eyes of his setting themselves on what was his. "What a powerful demonic aura," he mumbled, setting his book on the table, "the strongest I have felt in a long time."

His hands began to reach for the boy, but in a swift motion, Vergil used his katana as a barrier between the man and the boy. "Touch him, and die. He is my tool, and you shall not touch him."

"As you wish," the man replied, picking up his book, "yet let us go to Temen-Ni-Gru. I have found a way to unlock the tower. Take your tool with you."

Useless but useful. That part of his aura no longer confused the half demon. He was useless in his aura, but useful in his abilities. He sheathed Yamato and picked up the boy, who clung to him like a lost child as soon as he was in his arms. It made Vergil uncomfortable, but he ignored the feeling. The boy was nothing but a tool, and he was not to get attached. Attachments are what killed humans. He was just using this boy for his aura. That is the sole reason he saved him, and that is the sole reason he needed him. Once his usefulness was used up, Vergil would dispose of him.

A strange pang went through his chest as he thought of disposing him. It confused the half demon. He did not need the boy after he had served his purpose, and all useless things must be discarded. He was going to discard of the man after he had served his usefulness as well. This boy was no different than the man in terms of usefulness. Why did it bother him, the thought of discarding the boy, when the thought of discarding the man meant nothing to him?

Yeah, I know, I wrote the most cracky pairing in existence. I tried to keep Vergie-pants (yup, I call him that) as in character as possible, so I hope I was able to do him justice. I'll do my best to make this pairing seem believable and not something I randomly pulled out of the sky. Wish me luck on that...