The Plane of Euthymia is the world inside Raiden Ei's mind, where her consciousness currently resides after she discarded her physical form around 400 years ago... that was until Yae Miko found a way inside the Plane of Euthymia, which made the Plane of Euthymia lighter than before, once it was black and red and now it's a pretty light yellow and pink.. seemingly meaning that Ei is happy to see Yae Miko..

Yae Miko: "you missed me didn't you ei? its been awhile" she smiled at ei