"Our lives have ended, but we are not gone." Even in death, their pasts will not disappear. The former lives of the Shinigami, Arrancar and living humans. Major crossover
It's oddly, darkly hilarious to him, in all honesty. Of all the situations they could have ended up in, they end up in this particular one. A million to one chance that it would work out this way, and it does.
Aizen sits in his throne, the one he has stolen from under the King of Hollows, and he smiles in a genteel way as a thin, robotic-mannered man strides into his (his; no longer Barragan's, no longer the domain of hollows, but his) throne room and kneels like the pawn he knows he is.
In respect to their former roles in one another's past lives, it's very, very ironic.
"Ulquiorra," Aizen states his Cuatra's name without honorifics, because he doesn't need to bother and doesn't care, really. The Arrancar raises his head, and his vivid green eyes are empty, devoid of humanity and interest and all life. They may be green, but they're also coal black. When they first met, Aizen himself was floored at seeing the man (once his hollow mask had been removed, of course). He didn't let it on, didn't show it, but he instantly knew that this man, this pale recluse with his dark hair and his empty eyes, recognized him. Sosuke, Ulquiorra; their new names were of no importance. They knew one another, and they knew one another well.
None of the other hollows could ever know. They would never, ever know. Not because it was dangerous for them to know, but because it was a bit of an old shame for Sosuke Aizen; this man he had killed in previous times, but he had done it in a singularly cowardly manner, and even as he looked into Ulquiorra Cifer's eyes with no twinge of regret, remorse, or any emotion at all aside from smugness, he knew that if Ulquiorra could hate, then he would hate Aizen.
" Ulquiorra," He begins, nonchalantly, and if any others were to hear him speak to Ulquiorra like this (say, for instance, Gin or Tosen or Grimmjow), then they may be confused about how a living god could speak so informally, so easily, to a mere foot soldier like Ulquiorra. He was a good foot soldier, a very good one, but he was still just that: A lowly soldier. Ulquiorra stares blankly at Aizen, but there is knowledge, wisdom in the black depths of his emotionless eyes as he listens to Aizen.
"Ulquiorra, do you ever wonder how it happened like this?" Aizen asks, point blank, and Ulquiorra merely stares.
"How it happened like this?" He mimics Aizen's words, but there is no emotion behind them. He can't summon up something like emotion; doesn't have the capability to do that. "I suppose that it was fate, Aizen-sama."
"Fate?" Aizen asks, and almost laughs. "Fate is an excuse, Ulquiorra."
It's almost odd for him to see the sallow, pale black-haired man with no deep rings under his eyes, or without a mouthful of something sweet. It's very, very odd to see him sitting normally in a chair. But Aizen is exactly where he needs to be, where he deserves to be; on a throne. There is no notebook in his palm this time, no black-cloaked monster looming over his shoulder with an enormous smile and distinct, annoying hyuk hyuk hyuk laughter. There is only the tool of complete hypnosis strapped to his hip.
"Possible. But something decided it to be this way," Ulquiorra rebukes in monotone, rising to his feet. He may be subservient to this man, this total and complete monster that he's battled twice and lost to, twice, but he's no slave. He wasn't in life, and he will not be that in death. He will not bow down to a madman's will simply for the sake of bowing. And Aizen knows that, and he knows it well.
"Dismissed," Aizen waves him away, but a once magnificent detective is already walking towards the door, stuffing his hands in his pockets and hunching over ever so slightly, before straightening up. Habit. He needs to break himself of it, the same way that a mass murderer sitting in a throne at his back needs to break himself of that horrific habit of gloating when he knows he's won.
Light Yagami may be the ruler of all three realms, but L Lawliet will never shatter for him.