The Return of Thanatos
by HåvardWhat if we went back to the Gaz2 version where Thanatos indeed had been driven away from the Multiverse when Nithia was destroyed. His accomplice Arik was imprisoned for the crimes those two committed by utterly corrupting this civilisation to the point where the Immortals felt that it would have to be wiped from the face of Mystara.
Thanatos might still be able to supply spells in secret to his followers, though he is no longer part of the immortal hierarch and Hel assumes the position as Ruling Hierarch of Entropy (as per WotI).
The HWA Trilogy could then be placed in the context of the Return of Thanatos. As OldDawg stated elsewhere, the trilogy is flawed, but this can be remedied:
1) Thanatos manages to trick all of the immortals into travelling back in time with no chance of returning: In HWA1 Thanatos persuades the immortals to join him for an expedition back in time. What if instead, Thanatos appears and is attacked by a joined force of all the major immortals. He apparently flees back in time, though in fact this is a decoy, leading the immortals into a trap where they cannot return to their own time. Since not necessarily all immortals will be trapped in this version, not all clerics will lose their spells. Nor does Asterius need to play the specific role that he does, but he can. The most important problem should be that the remaining immortals do not know where the others are. Thanatos should have some way of preventing them from learning this.
2) PCs foiling Thanatos' plan. The PCs release Asterius from his imprisonment which again leads to Thanatos' plan being unravelled. I don't have a problem with this part so much as the next where the PCs, imbued with power from Asterius, and later other immortals as well are able to defeat Thanatos. Instead it would be better to have the PCs free a group of immortals and allow these to eventually fight Thanatos. This lets the players to get involved with the action without completely ruining the idea of Thanatos as the ultimate bad guy. Thanatos should be allowed to escape at the end, so he can return to make things interesting another day. More Marvel Comics than Tolkien, but still fun.