Quote from Azjenco
Well, it's mostly unimaginable because he doesn't actually know where they were sent, he merely speculated that it's a horrible place, a Void of sorts.
What's so terrible about "the void". I always see that in video games/lore about how terrible nothingness is. At least you don't have to look at ugly people.
And I'm pretty sure they can resurrect any baddy from Diablo's past. Nothing is in stone.