Crosis was one of the rogueBorg drones loyal toLore, acting as theandroid's second-in-command.
In2369, Crosisbeamed aboard theUSSEnterprise-D fromhis vessel and allowed himself to be captured so he could convinceData to join the group under the leadership of Lore. Previously, Data had experiencedemotions 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 theemotion 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 rogueBorg Collective.
During an attempt to rescueGeordi La Forge andJean-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, whenWilliam T. Riker andWorf arrived on the scene and Crosis was shot and killed. (TNG: "Descent", "Descent, Part II")