Crosis was one of the rogue Borg drones loyal to Lore, acting as the android's second-in-command.
In 2369, Crosis beamed aboard the USS Enterprise-D from his vessel and allowed himself to be captured so he could convince Data to join the group under the leadership of Lore. Previously, Data had experienced emotions after killing a Borg drone. Unknown to him at the time the emotion was not his own, but had been fed to him by the Borg, who were linked to Lore and the emotion chip he had previously stolen from Data. Crosis used the memory of the fleeting emotion to convince Data to follow him and defect to the rogue Borg Collective.
During an attempt to rescue Geordi La Forge and Jean-Luc Picard from Lore, a fight broke out among the rogue Borg. Crosis appeared to be about to triumph in a struggle with the leader of a rival faction, Hugh, when William T. Riker and Worf arrived on the scene and Crosis was shot and killed. (TNG: "Descent", "Descent, Part II")
Crosis was played by actor Brian J. Cousins.
The Descent first draft script calls him Cresis.