The only things I can remember about it are:
• The first (and only, last I checked) parts involved Celestia getting possessed by a demon
• It is revealed that Luna knows the demon, and implied that it played a part in this AU’s version of Nightmare Moon
• The demon reveals itself to Luna by using a private name between them: Shiarra, I believe, or similar.
• Somehow, the demon and its master intend to “save” (?) the world from some evil by getting rid of the sun.
• The demon and its master are either from the future or the past, but I’m not 100% certain which of the two, something about how Luna and Celestia as Alicorns are different from the Alicorns that the demon is familiar with.
•The last time I saw the comic, Celestia was in a Battle In The Center Of The Mind with the demon, but I don’t think the result was determined before I forgot about it. Should also point out, this was a few years ago and I’m only remembering it now.
One of the reasons I want to find this comic is because I’d completely forgotten the demon’s name. I dunno if it’s here or elsewhere, and I got bored of sifting through a “princess luna, comic, safe” search in reverse trying to find it.