Radiance
An element mixed with a pole: fire quickened into pure light, and the source of true daylight.
Fire quickened by living force until only the light remains. The Elder named this plane at the Twisted Grove in S3E38 and called it the source of the true-daylight light-crystal, the first crystal recovered from a Mól working ever traced to a plane rather than to a technique.
The empire was not merely binding elementals; it was drawing on the bright pole directly, and the light-crystals scattered through Idir are pieces of a plane no living scholar had cause to name.
The seat is empty. The god who held it has been quiet longer than memory serves, and few stories remain of who they were. No death was ever written down for them, only an absence old enough that the faiths stopped keeping the name.
Radiance stands opposite Ash, and the Elder named them together. Hikāru sits in one. The other has been dark for as long as anyone has counted.