"Locutus" is the name bestowed upon Starfleet captain Jean-Luc Picard when he is captured and assimilated by the Borg. In the Star Trek: The Next Generation episode The Best of Both Worlds, a Borg ship invades Federation space with the intention of attacking Earth. To properly facilitate humanity's assimilation, the Borg seek out Captain Picard of the USS Enterprise-D, intent on using him as a liason between them and humanity. Despite the Enterprise crew's best efforts, Picard is abducted and assimilated, transformed into the Borg's representative: Locutus of Borg.
By assimilating Picard, the Borg gained all of his tactical knowledge and experience and were able to adapt to Starfleet weapons and tactics before engaging them. Under Locutus' guidance, the Borg cube headed towards Earth but was intercepted by a Starfleet task force of 40 ships at the Wolf 359 star system. Starfleet's resistance proved futile against the might of the Borg and almost every ship in the fleet was destroyed, with over 11,000 lives lost.
The Enterprise-D would eventually catch up with the Borg at Wolf 359 and initiate a plan to rescue Captain Picard. Playing to Picard's expectations, acting captain William Riker's plan involved a more radical course of action, one that Picard would not consider. The plan was successful and Locutus was abducted from the Borg cube and returned to the Enterprise, where Lt. Commander Data attempted to access the Borg's collective consciousness via Picard's neural implants. Data managed to establish a connection with the Borg hive mind and Picard as well and attempted to plant a command into the network as the Enterprise engaged the Borg in Earth orbit. During the attempt, Picard's personality resurfaced and suggested "sleep", which Data interpreted as a command to order all the Borg drones on the cube to enter regeneration mode. The command was planted successfully and the Borg's attack ceased, but the malfunction caused by Data's command triggered a massive power feedback that caused the Borg ship to self-destruct. As the Borg cube exploded, Picard was severed from the collective and would later have his implants removed, restoring him to his normal self.
Although Captain Picard made a full recovery from his experience with the Borg, he would forever remain haunted by the memories of the atrocities he helped the Borg commit as Locutus. Worse still, when Starfleet engaged the Borg again years later, Picard would realise that he maintained a psychic connection with the collective. He would also come to learn of his true purpose as Locutus when he met the Borg Queen on his ship: he was to be her counterpart, a partner of sorts that would share her control over the collective. Because he would not surrender to her willingly, his assimilation was forced and he was turned into just another drone.