Old School informed GURPS Dungeon Fantasy gaming. Basically killing owlbears and taking their stuff, but with 3d6.
Is that a sort of D&D Call of Cthulhu mash up? I would love a monster book of eldritch horrors to fight. Hint hint.
More D&D Gonzo than D&D+CoC. See this review of the original adventure it's expanded from:Knockspell #5I'd write a monster book full of eldritch horrors to fight, but the emphasis would be on "to fight" not on "horror." In standard DF, flying polyps and byakhee and mi-go are things you fight and kill, not things that break your sanity.
As long as there are tentacles and psuedopods that writhe a book of eldritch monsters would be really cool. Weird powers would also be great. The underworld in the Vault of the Drow was always a favorite of mine. Maybe the primordial alien gods and their minions came from the underworld instead of outer space.