This is basically an original take on Darkside from Kingdom Hearts. There's a lot of involved backstory, concerning why he is no longer three stories tall and skeery, but... in a nutshell, he regained his heart at last, though it is (to say the least) rather worse for wear. He's still Darkness' pawn, though, if only packaged in a nicer box.
And yes, he carries a Dark Keyblade. Skeery, huh?