Maelstrom

Infinite and ancient, the Maelstrom spawned the other Outer Planes in cosmological prehistory and now surrounds them like a vast ocean of raw, seething quintessence, its tides forever gnawing at their edges. Where it borders another plane its structure takes on that plane's characteristics, though unpredictably and chaotically. Beyond those borderlands it reverts to its true nature: a trackless, liquid infinity devoid of stability and permanence, where serpentine proteans create and destroy with profound frivolity. The proteans are, paradoxically, organized into discrete choruses, each with its own philosophy and goals.

Swirling with oddities and wild magic and bereft of laws or structure, the Maelstrom also serves as a conduit between the other Outer Planes, used by armies of extraplanar beings, by hordes of demons spilling out of the Outer Rifts, and by the protean choruses that seek to return every other plane to the freedom of the chaos they emerged from. Sufficiently powerful magic can stabilize that chaos for a time, allowing demiplanes and stable islands to form, including a massive planar trade city that drifts through it.