Nyx is a character from "Past Sins" fanfiction https://www.fimfiction.net/story/41596/past-sins She is reincarnation of Nightmare Moon. The picture in not an illustration for the fanfic and is not related to the story just adult version of the character.
She still has her queen bed from "Nightstone Castle". It's too big to sleep alone. Join for the night and the night will last forever!