Image imported from derpibooru 
Original Description:
 
Another commission made by SakuraFaith
 
So for those that don’t know the markings on Selune’s body are part of the Nightmare that infected Princess Luna when she turned into Nightmare Moon. My headcanon for it is that it’s part of some Outer God/Elder God taint of madness that seeks to take over Equestria. Selune Darkeye here is among the few that has fallen victim to one of those otherworldly entities. Fortunately she was stopped before she fully became the “Gate” for one of these gods(the equstrian equivalent of Yog-Sothoth). Her markings were the taint turned into seal by Princess Luna’s subconscious when her moonlight shone on the solar winds of the Aurora Borealis.
 
Here’s a quote for those that want to know more about the specific kind of god that Selune hosts. I used Abigail Williams from FGO who actually was one of the girls that was convicted of “witchcraft” in the Salem Witch Trials.
 
“Yog-Sothoth knows the gate. Yog-Sothoth is the gate. Yog-Sothoth is the key and guardian of the gate. Past, present, future, all are one in Yog-Sothoth. He knows where the Old Ones broke through of old, and where They shall break through again. He knows where They have trod earth’s fields, and where They still tread them, and why no one can behold Them as They tread.”
 
Yog-Sothoth is a cosmic entity and Outer God. Born of the Nameless Mist, he is the progenitor of Cthulhu, Hastur the Unspeakable and the ancestor of the Voormi. He is also the father of Wilbur Whateley.
 
If you see on her head the eye-mark is changed to an eye within a lock. Signifying the change to her becoming the “gate”. The markings no longer hold back the corruption. Beware beware… for They are unleashed.
 
“Tainted” version:>>2716653(/images/2716653)