This idea has been stuck in the back of my head for a few days now, so I thought I would write it up and see where it goes. It's not quite a seed for campaign, but maybe getting it down on paper will solidify it.
The Mad God
It exists in a void of blackness and empty space, god of a dead world, unhinged by the loss of its followers and the slow decay of its realm, which was destroyed long ago in some forgotten cataclysm. Its divine power curls and twists in the void like so many black and terrible serpents, constantly seeking that which a god desires most. Worshipers.
The Mad God sends shards and fragments of its own being across trackless space and twisted dimension, seeking sentient creatures to make its own. It cannot challenge the gods of other realms in their own places of power. Instead it nibbles at the edges of their worlds, the lost places, the hidden tunnels and caverns in the depths, the sunken wrecks of galleons, and forgotten cities long covered by jungle vines. When the dark reflections of the Mad God touch these places they are altered, becoming conduits to its own dead realm. Those that explore these forgotten sites sometimes find themselves traveling through a strange reality inhabited by races and creatures from unknown worlds. The Nexus.
An aggregation of lost places drawn together by the Mad God, the Nexus is an endless maze of black caverns, tunnels carved by unknown hands, and lost cities crumbling to ruin; the forgotten places of a thousand worlds. There is no sky in Nexus, only dark vistas and endless depths. Those that find themselves in the Nexus are often lost forever, the conduits to their own world shifting and vanishing in the endless maze. Some despair and lose hope. Others struggle on, seeking a way back to their own realm and former life. Some chose another path and begin anew in this strange and terrible place.
For there is hope here. Islands of light and stability in the endless night. Conduits between worlds, once established, never entirely vanish, allowing powers from other realms to manifest in this place, diminished perhaps, but never lost. Wizards of lost worlds light the darkness with powerful alchemical spells while priests intone rituals to distant gods and warriors defend these new realms against the encroaching night. Disparate races gather together in common purpose and form communities bent on one goal: survival.
But there is despair here too, for between these points of light lie many hazards. The power of the Mad God touches only the forgotten places of distant realms and most of those regions were forgotten for good reason. There are dangers in the darkness, some mundane, some so terrible they exceed the boundaries of sane thought. Dark things creep from tunnel and cavern, seeking to destroy the islands of order in the middle of chaos.
And the Mad God remains hidden in the center of it all, a vortex of need and hunger driven by endless desire.