Avery III was a Delta Quadrant planet riddled with caverns where the Vidiians ran a hidden facility, and where the scientist Sulan split B'Elanna Torres into human and Klingon halves.
The planet is the setting of the first-season episode Faces. A Voyager away team surveying its caves is captured by Vidiians, who use prisoners as labourers and as a supply of replacement organs.
The chief surgeon, Sulan, believes Klingon physiology may resist the Phage. He uses a genetron to separate Torres into two people, one fully Klingon and one fully human, and infects the Klingon with the disease to study her. He also kills Lieutenant Durst and grafts the man's face onto his own.
Chakotay, disguised as a Vidiian, helps the two Torreses escape. The Klingon half dies protecting the other, and the Doctor later restores B'Elanna's mixed heritage.