According to the Loveletter from the Fifth Era, the cycle of subgradiance goes Void to Aurbis to Aetherius to Oblivion to Mundus to Mortal Death to Z. Presumably, Mortal Death refers to the Dreamsleeve. The Loveletter mentions little about this "Z", only that as Mortal Death is an Echo of Oblivion which is an Echo of the Void, so is Z an echo of Mundus, and that reaching this subgradient reveals CHIM. What could this strange dimension be?

Maybe it is like the Mantella Crux.

Z is the end of the Alphabet

It's the end of everything, starting at Mortal Death.

Whatever the [VEHK] any of this means.