Severitus
A fanfiction story challenge made by the author Severitus on fanfiction.net. It includes a very specific set of requirements, found at their LiveJournal page (google "severitus challenge livejournal").

Requirements:
1. Severus Snape must be revealed to be Harry's father.
2. Remus Lupin must have returned to Hogwarts for some reason or another (unless it's a 3rd year AU, in which case he's already there).
3. Harry must undergo some progressive physical change starting on his birthday. No *tada!* and suddenly he looks like Snape.
4. The story must be based mostly around Harry and Snape.
5. (Optional) Incorporate one or more of the conversations posted on the LiveJournal page. Change them however you want (it's encouraged!).
6. Make a note somewhere that it's in response to this challenge, so all the other nice people can give it a shot too, and be able to find the other challenge responses.
7. Tell Severitus that you wrote a response to the challenge! Feel free to either email them or let them know in a review on one of their stories.
I was reading a fic tagged as a Severitus, but the author didn't include all of the requirements of the challenge. They probably assumed that "Severitus" was just a fancy name for a "Mentor!Snape" fic, and never read the challenge themselves.