An Evil!Sue is a sub-type of Mary-Sue which is found in fanfiction. This type of Mary-Sue character is almost always on the dark side in a given fandom. She, like all Sues, is beautiful*, talented, powerful, and of course, annoyingly perfect.

She is usually created by a Suethor who eithor prefers or has a fascination with the villains instead of the heros/good guys in the famdom in which she is writing. Oftentimes, the Sue will either be related to a canon villain (i.e. Voldemort's daughter in the Harry Potter Series) or fall in love with a canon villain (i.e. Draco Malfoy also Harry Potter)

The Evil!Sue is somewhat like Goth/Punk!Sue in that she will usually have an angsty past which is why she's on the Dark Side to begin with.

*Some Evil!Sues have long flowing black hair, pale skin and X color eyes, they may wear black or red or any other color which could symbolize evil or darkness in the Suethor's opinion.