This is the designation given to Captain Jean-Luc Picard while he was part of the Collective in late 2366 to early 2367 (Stardate 43989.1 to Stardate 44001.4). The Borg Queen had wished that Picard would freely give himself to the Collective to be her human counterpart.
Picard, however, resisted, and was forcibly assimilated. Picard’s knowledge of Federation technology, defence strategies and weapons allowed the drone Locutus to lead an attack into Federation territory at Section 001. The subsequent battle at Wolf 359 lead to thirty-nine Federation ships being destroyed.
Shortly after this, Locutus was abducted by the crew of the Enterprise-D. Locutus’ link to the hive mind proved useful in defeating a Borg Cube. The medical staff onboard the Enterprise were successful in removing the Borg implants and returning Picard to his command.