Ryuunosuke didn't know what to think.

When he found that old occult book at his parents house, he was assaulted with such a wave of inspiration that he immediately went and did his joy and passion: killing.

Those macabre rituals in that book added an air of mysticism to the experience of death that he made a spree: killing unaware and defenseless people, especially families, and observe their reactions, any single moment, if repetitive sometimes, was precious in his path to understanding Death itself.

That and he thought it was 'cool', well 'cooler' than previous killing experiences, but 'cool' nonetheless.

So he doing his usual outing: this time two parents and an older sister. He left the little boy, gagged but alive, as sacrifice in order to test something new, a demon summoning ritual, or something.

Seriously, that book was so old that he can't completely understand it. Meh whatever.

After making a magic circle with the deceased's blood and chanting the ritual, a light, mixed with some purple, flashed through the circle.

After it dissipated, there was someone in there: he had some kind of organic-looking red and black bodysuit that gave him and edgy air, and his face was covered by a black glass helmet.

And if that weren't enough, there was some kind of wrongness on him like he wasn't supposed to exist at all. Well, and any magic sensitive would just notice the Darkness he exudes even if subconsciously. Those factors just scream danger to anyone, especially the child who's just squirmed much more in presence of that abomination.

But Ryuunosuke didn't think of the possible danger, just thinking about how cool the ominous-looking man could be. Especially when he summoned him with a magic circle, s0o he must be a demon!

How...coooooooool!

"Hey" a deep, if youngful, voice came out of the masked man "Are you the one who summoned me?"

Oops, Ryuunosuke forgot. Just where were his modals? That demon just got out of Hell so he has to demonstrate hospitality, even if it wasn't his house to begin with!

"Uh… yeah." Smooth, Ryuunosuke, smooth "My name's Ryuunosuke Uryuu. I'm employed at various part time jobs, though the rest of the time I prefer killing. My favourite victims are children and women. And I was experimenting with black magic to add a 'spark' into my murders. So, umm..."

No one of them talked. God that was awkward. The murderer wasn't especially a social butterfly though he at least knew how to function in society, he didn't expect to talk with a true demon of all things. Then he remembered that kid he bound to that chair and was trying to liberate himself.

"How about we talk over a few snacks? I don't know exactly what you demons prefer but I think you at least like human souls or flesh." Ryuunosuke offered, which made the kid despair even more "Wait, do demons decant towards sinners or innocents?"

The masked guy looked towards the child, who sensed a overwhelming sentiment of indifference from the former, even if he had his face covered.

Then he looked towards Ryuunosuke, then to the arm that apparently had some glowing tattoos. "Nope" he summoned his weapon, some strange red and black sword apparently made from gears with chains wrapped around it, a pair of draconic looking blue eyes located both in the shaft and the tip. Strangely enough, it resembled a key of all things.

"I have a better idea."

Ryuunosuke didn't have time to formulate a question when he heard, rather than felt, his arm falling to the ground. The cut being incredibly quick and clean to process it, even the blood spray was delayed.

He also didn't have time to respond to that when an arm grabbed his face and he knew no more.

Void Grail

'Good grief' thought Vanitas. It wasn't enough that his asshole of a teacher abused him all his incomplete (and somewhat) short life in order to wield the Darkness so that he could forge the x-blade, he had to die when this idiot of a goody-two-shoes brother destroyed the x-blade when it obviously would shatter their Hearts.

Now, the Dark Half was somehow revived for some kind of war, not unlike the Keyblade one, due to fighting over a powerful mystical artifact with access to the Heart of all creation. The difference being that instead of Keybearers, seven mages summon seven spirits or Servants, which supposedly were famous heroes from the past, and were bound to their Masters in order to fight in proxy fights. The prize, for both Master and Servant, is the wish granted to them by that same artifact with access to the source of anything in existence.

Pfff. That's got to be the stupidest thing he's ever heard of.

Why fight for a single wish when you have the opportunity to seize the entire universe? You could anything you want! Like creating an even greater war! Seven pairs of participants? Get real!

By Kingdom Hearts, this World's mages must be incredibly stupid. How did they manage to survive?

From the information he obtained, those magi are worse than Master Xehanort, both in character and skill. Even the old man admitted the power that technology was able to accomplish, especially in some very advanced Worlds.

And Vanitas expectatives for the local magi lowered even more when he learned that he was summoned by a fucking murderer, that apparently didn't expect anything from the ritual he apparently executed.

Welp, Vanitas was a pragmatic at heart, so he took advantage of his Master's ignorance to cut the arm where the Command Seals were located, as well as doing a favor to that World by burning that filth of a man into dust with a Dark Firaga.

Overkill? Maybe. But god did that felt good!

And the good news is that he doesn't even need a Master to sustain himself. His skills apparently letting him to generate prana from negative emotions alone.

He was tired of being someone else's pawn. He was his own Master now.

Just in case, he would hold on those Command Seals. Maybe he can use them in a ditch or just use them to summon his own Servant.

Wait, isn't that against the rules?

Who cares, rules exist to be broken!

Just as Vanitas was about to fantasize about having a powerful slave that can rival his most powerful Unversed, as well as Keyblade Masters, at his beck and call, he heard something falling to the ground.

Oh, right, he forgot.

That child, in his attempts to escape, accidentally upsetted the chair's balance and fell, still bound to the chair.

Ouch.

Vanitas slowly approached the kid, deliberately making noise with each step. Then he knelt down, now level face with the confused, pained, and just terrified kid.

"Boo." With that simple sound word, Vanitas made the boy's eyes to whiten out from the shock, making him unconscious. If he hadn't had that gag, his mouth would be foaming. Though the odor of crap and urine was palpable.

Blegh. What a crybaby.

Well, at least his despair made for a nice treat.

Vanitas then wished for the tallest building of the city, all while willing a Dark Corridor into existence that engulfed him.

Now, at the roof of Shinto's only skyscraper, the Dark Keybearer let out a chuckle which rose into a laugh which further devolved into that of a demented madman.

"I may not have my Keyblade War." He let out with a psychotic smile "Then I have to content myself with this Holy Grail War!" He held away his arms from his body, presenting himself to Fuyuki City like a showman himself.

"And I'll have my desire of destruction granted." He observed his right hand, the one with his Command Seals, which shone through his covered limb: two skeleton keys crossed over like an X, inside a Heart's contour and a crown at the top.

The Dark Keyblade Apprentice Vanitas, Servant Caster, has officially entered the Holy Grail War.

Void Grail

"Let's hope the author updates this story more often than the other one." said the Master of Masters while comfortably lying down on a coach, popcorn and drinks ready "It was a pain searching for Vanita's Heart in an alternate timeline and putting it into the Grail. Though I forgot to invite Zelretch, maybe I'll send him the record once it's over"

"Now, let the chaos, begin."

AN: First of all, let's give our respects to Kanimize, creator of Kingdom Hearts stories with great potential: Fate No Heart and Empty In Remnant, which follow Xehanort participating in the Fourth Holy Grail War and a reincarnated Vanitas in Remnant respectively.

I always thought of Vanitas as a very interesting villain, especially one mentored by one of the greatest chessmasters of gaming. Not to say he would be perfect for the Nasuverse, especially in a story created by the Urobutcher himself, so a universe as dark and gray as this makes it perfect for my first villain fic. So that he could derail everything with his powers, personality and cunning. All hail the Unversed King!

And yes, he will have a Servant, Avenger, due to, you know, obvious reasons. And there will probably be an Alter at some point…

Here's Vanitas Servant sheet:

Caster-Vanitas

Strength: B

Endurance: C

Agility: A+

Mana: A

Luck: D

NP: B

Skills:

Avenger/Self-Replenishment (False)-A: Being a creature of pure negativity, Caster can feed on pure negative emotions and obtain magic energy from it. Even a lingering thought is enough to replenish a moderate amount of it. It's so much the efficiency and quantity of energy collection that he could emulate Independent Action with nearly rank EX with a lifetime of negativity.

Territory Creation-D: Even while being a Caster, Vanitas mainly trained on offensive magic while moving around Worlds with his master. Because of that, he hadn't ever thought of making his own workshop, much less study magic for scholarly purposes. Though he can improvise a base of operations if he has to stay for extended periods of time in a World.

Seeker of Darkness-B: Being trained by a Dark Keyblade Master and having a dark Heart himself, Vanitas has extensive knowledge of the dark arts, an example being the Corridors of Darkness, extradimensional passages that allows for instantaneous transportation to near any place imaginable. This skill also has the benefit of allowing its user to use spells or rituals near instinctively as long as it involves Darkness, the Keyblade also helping in simplifying the process enormously as long as one's Heart is determined. Gives also a bonus to Darkness related magecraft and attacks.

Noble Phantasms:

Keyblade: Void Gear (Anti-unit; A+): A conceptual weapon made from one's heart, each Keyblade is a copy of the original x-blade that guarded Kingdom Hearts, an equivalent of Akasha in the infinite omniverse. The high rank comes from the Mystery that surrounds the Heart and the infinite possibilities (Light, Darkness) it possesses. If it were the original x-blade, it would be ranked EX. While this Keyblade isn't one of the most powerful ones, it has great reach and huge physical attack, though it lacks a bit in magic potency, despite its user proficiency with elemental attacks, and it's a Darkness-aligned weapon, making it very compatible with its wielder. The Keyblade is able to lock and unlock any lock, physical and metaphysical, including even the Hearts of Worlds. The Keyblade is also a natural conduit of magic making it an ideal foci for spells and despite most of them lacking a cutting edge, they can be impossibly sharp. A Keyblade, a somewhat sentient weapon, chooses its own owner, making it nearly impossible to steal unless the usurper has an exceptionally strong Heart or are already Keyblade Wielders themselves. There are even more functions unknown about it that the user may discover, whether by luck, practise or pure determination.

Unversed: Those who are not well-versed in their own existences(Anti-unit/army; A-E): An ability Vanitas was already born with from the moment he and Ventus were separated. The negativity produced since by that unholy disunion manifests physically into monsters of differing strength and abilities. They don't even require too much prana to create, only strong emotions and feelings, of which Vanitas can extract from himself or others near infinitely. All of them maintain a connection with their master more profound than normal familiars, so every slain or dismissed Unversed returns to him, always. That includes their pun though he can ignore and using the negativity of that pain to produce even more Unversed. And if Vanitas were to perish, the Unversed will also disappear, though it's possible for an Unversed to survive if exposed to huge amounts of negativity that border on unnatural levels.

Oh, and Vanitas can summon various of his Remnants *evil laugh*