Locutus of Borg was the Borg designation forced upon Starfleet Captain Jean-Luc Picard after his assimilation in late 2366. The Borg intended to use Picard as an intermediary, a spokesman for the Human race in order to facilitate the assimilation of Earth so that the process would be as quick and efficient (or as perfect, from the perspective of the Borg Collective) as possible, with the fewest number of casualties on both sides. (TNG: "The Best of Both Worlds")
The Borg Queen herself had been on board a Borg cube where Picard had undergone his transformation into Locutus. Interested in overseeing this event, she had intended for Picard to become her equal counterpart but, when he refused to give himself willingly and accept his assimilation, the Queen was forced to turn Locutus into merely a drone, of which there were already many others. (Star Trek: First Contact)
Picard's assimilation allowed the Borg to acquire the whole of his knowledge and experience, as well as his own personal knowledge (a fact that was made apparent when Locutus addressed Commander Riker as "Number one"). Picard's detailed information regarding Federation technology and strategy yielded the Borg a significant tactical advantage when Starfleet confronted the Borg cube at Wolf 359. (TNG: "The Best of Both Worlds, Part II", DS9: "Emissary")
This access proved two-way, however, as the crew of the USS Enterprise-D was able to capture Locutus and use his link to disable and destroy the Borg vessel by sending the Borg cube a command to regenerate, creating a feedback loop that destroyed the cube and severed Picard's link to the Collective. (TNG: "The Best of Both Worlds, Part II")
Though his implants were removed and his wounds were allowed to heal, Picard's assimilation continued to haunt him. He returned to Earth, paying a visit to his family in La Barre, France, where he eventually came to terms with the ordeal. (TNG: "Family")
Despite his separation from the Collective, Borg drones that had never encountered Locutus, such as Third of Five, still referred to Picard using this name, due to Locutus' experiences remaining within the shared Hive mind. (TNG: "I Borg", "Descent")
When Benjamin Sisko made contact with the Prophets in 2369, one took the form of Locutus as he appeared on the viewscreen of Sisko's ship, the USS Saratoga, shortly before the Battle of Wolf 359. (DS9: "Emissary")
When the Borg attempted a second invasion in 2373, Picard's experience with the Borg not only gave him prior warning of the attack, but also allowed him to pinpoint a weakness in the Borg defenses, resulting in the fleet destroying the invading vessel. (Star Trek: First Contact)
When Captain Kathryn Janeway was conducting negotiations with the Borg in 2374 during the Borg-Species 8472 War, she requested to speak to an individual, citing to the Borg, "You've done it before, when you transformed Jean-Luc Picard into Locutus." (VOY: "Scorpion, Part II")
Locutus was played by Patrick Stewart. Regarding how he portrayed the character in the two-parter "The Best of Both Worlds" and "The Best of Both Worlds, Part II", Stewart explained, "The interesting situation for me was how to create this murderous, autonomous figure while retaining, behind all that, the shadow of Jean-Luc [....] I think, for me, the Borg episodes were not as dramatic as they were for our audience. I can think of other episodes in which I felt that the character was expanding, developing and learning much more than in that one. It principally for me was a way of trying to find out how technically to make the Borg character work. Of course, a lot of it was left to the makeup department and the special effects department. The makeup made a significant contribution." (Star Trek: The Official Fan Club Magazine issue 87, p. 4)
There is a noticeable difference in the makeup design of Locutus between the TV series and Star Trek: First Contact. Among the changes seen are airbrushed "wiring" on the skin (or beneath it), slightly more detailed Borg implants and a completely new body suit. The changes enhanced the somewhat sparse original look and corresponded with the "new look" of the Borg for the movie.
In the comic story Star Trek: The Next Generation - Perchance to Dream, the Enterprise crew were attacked by a telepathic weapon called the Chova, which forced its victims to experience dreams and hallucinations focused on their personal failures. However, it was discovered that those with Multiple Personality Disorder could render the Chova inert (The multiple personas overwhelming the Chova, which could only attack one personality at the time), so Picard was deliberately infected with the Chova, as his mind meld with Sarek, the probe that gave him the memories of Kamin and the remnants of Locutus still in his mind gave him the makings of an MPD. The four defeated the Chova, but Locutus then attempted to regain control of Picard's body, nearly "killing" Kamin and Sarek before Picard gathered the mental strength to stop Locutus.
In the comic story arc The Worst of Both Worlds!, the Enterprise crew gets pulled through a rift into an alternate universe in which Riker's plan to rescue Picard from the Borg fails and the Borg have conquered Earth. The crew teams up with their counterparts, whose Enterprise is only the stardrive section, as their saucer section had to be abandoned. The two crews managed to capture Locutus and use his link to the Collective to defeat the Borg, much like in the canon universe, except the "sleep" command couldn't be used. Picard gambles on the possibility that his alternate self also mind-melded with Sarek, like he did in the episode Sarek. Whispering Spock's name in Locutus' ear causes the Vulcan emotions to trigger and allows his humanity to break through. Using the alternate Picard's suggestion of "eat", Data is successful in immobilizing the Borg and saving both Earth and the alternate Picard. It is later revealed that he was responsible for opening the rift which pulled the Enterprise to the alternate reality.
In Star Trek: Armada, set in 2376, a new Locutus of Borg was created by the Borg that had captured a Dominion cloning facility. This Locutus led an attack upon the Federation and the Klingon and Romulan empires in an attempt to gain an Omega particle, resulting in the assimilation of Spock (who was on his way to a Romulan-Klingon peace conference) and the conquest of Earth. However, aided by the USS Premonition, a time-traveling ship from the future under the command of Captain Thaddius Deming, the USS Enterprise-E was able to undo this Borg victory, and the Locutus clone was destroyed after a brief confrontation with the original Picard.
In the novel Resistance, set in 2380, Picard became Locutus once again in an attempt to defeat the Borg Queen and neutralize them. After having defeated the Queen by re-altering her into a normal drone, Dr. Crusher was able to restore the captain to his normal self.
During the novel The Return by William Shatner, it is revealed that Locutus is not as unique as he has been portrayed; according to this novel, the Borg commonly assimilate a specific individual from a new species to act as a "speaker" to the new race to try and convince them of the merits of assimilation, thus making the process of assimilating a species easier and less 'wasteful'. During the events of the novel, the Borg create a Romulan "speaker" known as Vox, who subsequently proposes a Borg/Romulan alliance as a means of effectively conquering the Federation and the Romulan Empire, secretly intending to assimilate the Romulans the first chance they get. The Borg also attempt to assimilate Spock, thus turning him into their "speaker" to the Vulcan people, but due to Spock's past mind meld with V'Ger – an aspect of the Borg Collective – the traces of the Collective in Spock's mind from the meld result in the Borg assuming that Spock has already been assimilated, and thus leave him alone.