DC reveals a new dark origin for the Joker of Batman: City of Madness as an Eldritch monster tied to Arkham. The compelling aspect of the Joker's multiple origins adds to his cosmic horror appeal.

or the 1994 movie In the Mouth of Madness - just two examples of the plethora of resources available. While planning their cosmic-themed D&D adventure, it is worthwhile checking out Chaosium's ...