Illumina is a Rift Plane where Light and Darkness reside.
Summary
After tracking down the Keepers of Light and Dark (who happened to be Gade and Cyanea, respectively), they take Raven to Illumina in order to awaken the elements to take physical forms. After a battle to claim 5 territories for the Light, it manifests into a physical embodiment of the element, Lumino. Once he is defeated, Raven has to claim 9 territories for the Dark in order to awaken Tenebria, the embodiment of Darkness. Raven is able to defeat her in battle, but this causes the Rift Plane to collapse. With guidance from Gade and Cyanea, Raven manages to escape from the Rift Plane.
Geometry
The layout of Illumina is incredibly basic; it is simply 5 to 9 large platforms joined together by what appear to be extremely thick vines. With 5 platforms, it is arranged in a diamond shape. The path in which Raven must use to escape Illumina is littered with spikes that can damage her if she strays into them, though Gade and Cyanea will clear a safe path for her to use.
Story Appearances
Illumina appears in only one episode of the Raven's Choice DLC.