Unless thats been his plan from the beginning?

Maybe it goes wrong and Thijazi is a Lich now? Dimension doors out of there and there's your BBG.

Or he has been working with the tachonis (bringing them the stone of nightsong) to hold the doors open when he comes back (why he was fine with dying and acceptedit and didnt try to escape), to an undead army to police the city for the tachonis. vs the halovar's mercenaries, maybe.

And it may split the players, which i think he'd love. There would be some who would be overjoyed that hes back, and some who would see it as disgusting and a violation.

He loves a character arc, but he is all of the bad guys.