Would he die or would he never be able to transform into his true form?
In diablo_verse, I was under the impression when their physical bodies are destroyed then their souls return to hell. I am not sure if they can just destroy the soul. Although uldyssian has wiped lucion out of existence in the lore. So there’s probably a way, would probably need blizzard to come up with the means unless they somehow bring uldyssian back to accomplish the feat.
Which makes 1 wonder why he didn’t just wipe out the whole triune and cathedral out of existence in the first place. I guess we wouldn’t have to diablo_verse if that happened.
It’s never directly explained. My personal assumption is that he’d go back to Hell, since the whole soulstone business was a bit of a hoax in the first place. Uldyssian was a very special exception in regards to the permanent annihilation of a demon, and the handling of his abilities and the power levels are my greatest issues with the Sin War book trilogy. That said, if I had to assume and explain the contrivance, I imagine he didn’t use that ability again because the one time was a particular exception where he did it on reflex and his prowess simply multiplied from there, not necessitating the action again. Also, the ability was used with direct contact on a single individual - quite a different matter from obliterating, well, entire organizations at once, especially when his ultimate meta powers spawned at the end of the story (which nearly obliterated the world by itself).