HK-66 or HK Terminator was a class of deadly and heavy assassin droids and terminators who was created in 3,934 BBY and placed theirselves among Revanchists serving Darth Revan and later Keldevians. He was a ruthless warrior and impregnable terminator. He also participated at battles and served numerous Kelden and Humans. Darth Barlog had three HK-66 that executed everyone who disobeyed Darth Balrog. HK-66 was servant of Alexandra Keldaria and helped her escape from the Sith Base on Ziost.