Seems like they're preparing a new part of Time Temple.

I'm just digging through .xml extractions of a couple of the .wz files, so my searching methods are far from efficient, but I see no mention of the Black Mage being immortal.

 Spoiler

The Black Mage founded the town of Magatia in the hopes of using both kinds of Alchemy to achieve immortality, as far as i'm aware, it failed and he disappeared, but his experiments lie buried underneath Magatia in a secret room (which you can enter)

I think thats right at least, its been a while since i did those quests.

But i'm guessing that it must have been before he was sealed away, and the Heroes clearly believed they could stop him, so i doubt he actually achieved immortality.