Marasmos, Lord of Suppuration
The Planescape setting was ambitious, leveraging itself up as a meta-campaign that encapsulated all the other Advanced Dungeons & Dragons settings into a coherent narrative. That was no easy task, when some settings like Dark Sun and Spelljammer worked very hard at being isolationist. Even Dragonlance and Forgotten Realms tried to withdraw from the framework set in place by the Planescape setting. That reluctance was not reciprocated, as the writers of the Planescape setting made herculean efforts to fold not just the rules of the other settings, but their characters, plots, and history. However, some campaign worlds where exceedingly difficult to work with, especially the Ravenloft setting.