How would you stay an aspect of C'thulu? The players are about to embark on the final adventure of the campaign where they have to stop Morgoth from worming his way back into the world and triggering a prophecy of Armageddon.
I'm thinking of a C'thulu-esque scene with tentacles and chaos as he proves his way through an opening between the void and middle earth.
Pre-existing creatures are easiest to adapt but anything works.
Its an 11th level party and should be a difficult fight. PCs will die. It should be a hard-fought victory or barely a defeat.
I'm open to other ideas as well.