it was my nerdy understanding that sauron very likely cannot come back, but morgoth would likely return at "the end of the world"
unles maybe morgoth can pop a rez on sauron? like how much mana would it cost?
It was my understanding they put him in some kind of void that was (supposedly) impossible to get out of.
Wikipedia:
The Valar exiled him permanently from the world, thrusting him through the Door of Night into the void, and he is not expected to return until the prophesied Dagor Dagorath, when he is expected to meet his final destruction. His evil remained, however, as “Arda Marred," and his will influenced all living creatures.
Door of Night - Wikipedia, the free encyclopedia
Dagor Dagorath - Wikipedia, the free encyclopedia
Apparently the "Door of Night" looks like the Dark Portal from wow: