Living dead girls
Living Dead Girls is a fanfiction series authored by Psychotol.
This series crosses Daria over with Buffy The Vampire Slayer and Angel by having Illyria's sarcophagus begin accidentally sent to a shopping centre in Lawndale where Stacy Rowe was accidentally infected with Illyria's parasite form. It also has Jane being sired by a vampire, and Tiffany being an active Vampire Slayer, and Daria's likeness being imitated by an android.
The first episode was intended as a one-off feature, but expanded into a series as the author got ideas for it.
Other fictional universes crossed over with Daria and Buffy include The Bill, Ultimate Force, and Law And Order. This was done where locations support inclusion to reduce the amount of author input needed in the creation of fictional characters for said locations.