Fate
I think it goes without saying that this place is chuck full of spoilers.

X: The Destiny War - the movie
Like everyone in the “X” movie besides Kamui (don’t ask me what he’s going to do with himself after all that's happened), Seiichirou dies a rather violent and unpleasant death. It happens thus: leaving Yuzuriha with Inuki outside, he and Karen enter Tokyo Station to hunt down the Chi no Ryuu. They split up and Seiichirou is attacked by Nataku, who punches a hole through Seiichirou’s side. He's bleeding profusely but still standing, and they prepare to fight just as the scene switches to Karen’s final stand against Asagi. She destroys the station in a wild burst of flame but did make it out alive, and her kekkai falls. Seiichirou watches stricken from a distance. Later, Seiichirou and Nataku have made their way to the Sunshine 60 building. His kekkai is up and the roof top explodes. Standing amid the rubble, Seiichirou demands to know what it is the Chi no Ryuu are fighting for. Nataku replies that they want only to destroy the human race which pollutes and corrupts the planet. Seiichirou admits that what it says is true; however, he wants to protect the ones he loves. He summons up a whirlwind strong enough to tear up the concrete beneath their feet. Grabbing a hold of Nataku, Seiichirou vows that in order to repay his companion, he will not let Nataku live. The two plummet to their deaths 787 feet from the top of the 60 story building onto a highway overpass below. Slowly, Seiichirou’s kekkai disintegrates as the Sunshine 60 building collapses and the sun rises over what’s left of Tokyo.

X - the TV series
Working on it!

X - the manga
In case you hadn't heard, CLAMP is currently not working on "X". Nor have they finished the series yet. And so...waiting.

Hajime - Uchi