Crosis was amongst a contingent of Borg who were severed from the Collective after the re-assimilation of Hugh, a Borg that had been rescued by the Enterprise-D crew over a year prior. Hugh's newfound sense of individuality had spread across his branch of the Collective, infecting the others like a virus. Uncertain how to function as individuals, the Borg fell into chaos and fought amongst themselves until the android Lore discovered them. Over time, Lore taught the former Borg how to fuction individually and Crosis became the machine's right-hand man when Lore launched his campaign to destroy the Federation.
In "Descent Part 1" Crosis beamed aboard the USS Enterprise-D both as a diversion to give his ship time to escape from battle and to corrupt and retrieve Lt. Commander Data. Though shot and imprisoned, he activated an implant on his arm that emitted a carrier wave to Data, who was guarding his cell. The wave transmitted an emotional program to Data, overriding his sense of ethics and filling him with feelings of hatred and anger. Data then released Crosis from his cell and the two stole a shuttlecraft and escaped into a transwarp conduit, returning to the planet where Lore and his Borg followers were based.
In "Descent Part 2", when Data's ethical program was re-initialized, Lore decided that his brother had become a liability and decided to kill him. The errant Borg Hugh, who was mixed in with the crowd inside Lore's temple, suddenly emerged to stop Lore from shooting Data. Crosis then stepped in and pulled Hugh away from Lore. He was about to shoot Hugh when he was shot himself by Commander Riker, who sniped him from an opening in an environmental duct above the temple's common room.