Dark Avenger, dav
Dark Avenger was the pseudonym of a virus author supposedly from Sofia, Bulgaria. The name 'Dark Avenger' was derived from the string "This program was written in the city of Sofia (C) 1988-89 Dark Avenger" found within the first virus attributed to Dark Avenger, also known as 'Dark Avenger'.
Credited for coding the most infamous polymorphic engine ever, the Mutation Engine (MtE) in 1988. The MtE was used to generate polymorphic decryptors, effectively bypassing detection by the Anti-Virus programs of the day.
It has been suggested by some that the Dark Avenger personality was actually a social experiment, and not a single person but the collaborative efforts of 2 or more individuals, namely Todor Todorov and Sarah Gordon. Todorov and Gordon were two of the only people whom Dark Avenger was believed to retain personal contact with and knew his true identity.