Perish:
to die or be destroyed through violence, privation, etc
Actually, the whole mystery centers around the fact that nobody knows what happened. Every account is different. Vivec himself, a god, and IIRC, actually there at the time, didn't know for sure.
Official lore says all of these: they died, they became gods, they transported to oblivion or beyond, they were destroyed by Azura, etc.
There were quests in Morrowind, at least 6 books, conversations with gods, immortal mages, and the last dwemer. The result was that there was no answer, and you had to decide for yourself.