Killing named horrors?

[TwoBits]

If you manage to kill or at least defeat a named horror, is that unit out of the game, or can it return later?

I actually managed to put the kibosh on Hruvur, the Abomination of Desolation, a great fish-skull looking thing, thanks to a Ring of Warning and 10 exceptional bodyguards (8 of whom were killed before Hruvur packed it in), and I'd rather not have to see him again (yeah, someone has put up Astral Corruption).

Is he permanently gone, or can he come back?

And just how many of these uber-horrors are there (on the off chance that once they're gone, they're gone)? I almost managed to beat something called Agon, Eater of Dreams, before he managed to panic my commander (with his Fear 15!).

I'd like to think that if I managed to somehow to actually defeat all of the really nasty ones, Astral Corruption wouldn't be such a hassle. Seems unlikely though...

Anyhow, this is my first run-in with A.C., so any ideas on what to expect (I got kinda lured into a false sense of security, after I only saw one lesser horror the first two turns of the global), and how to deal with it, would be greatly appreciated!