Limbo is the outer plane of absolute chaos, home of the slaadi (the frog-creatures of chaos) and the githzerai (the schismatics of the gith, the reverse of the githyanki).
Its matter is pure chaos: matter, energy and elements in constant flux with no stable form. It is shaped by willpower, an art the githzerai master to raise monasteries that do not dissolve.
In the gesta
Limbo appears via the Tavern of the World Serpent, one of the planes that show through the tavern’s liminal space. It may cross with the March of the Modrons. It is the place where rules do not apply and the cradle of chaotic creatures, perhaps including some manifestation of the gem-world Eilidh (to be confirmed). It is present in the central cycle, among the Regulators and perhaps in Caldero.