Line the long empty corridor in Asmodeus’ Lair with a few statues.

Add a clear and obvious ice theme to Asmodeus’ Lair, as an indication to the player that he has nasty cold magic at his disposal.

Asmodeus and Baalzebub have a chance of re-appearing hostile to their levels (not a certainty; if they always came back there’d be no reason not to kill them off) when the player is on the way back up with the Amulet. Flavor-wise, this is because the player only paid for safe passage ‘‘down’’, and now the player has their coveted Amulet.