HK-51 series assassin droid

Despite the ban on HK-series assassin droids by the Galactic Republic, the Czerka Corporation started to develop and manufacture several HK-51s on Nar Shaddaa in secret during the late stages of the Great Galactic War. This order was for the reconstituted Sith Empire, who intended to deploy the new droids around a decade into the following Cold War.

Sometime after 3668 BBY, the Sith warship Fatality, carrying the droid shipment, was hijacked by a Dread Masters' apprentice, who attempted to land and deploy them upon the planet Belsavis to in a bid to rescue his captured masters. The Republic's unit stationed on the planet shot down the vessel, which crashed and sank into the frozen waters of Section X. Not only was the only known shipment of HK-51 units lost, but slicers of unknown affiliation managed to delete all schematics of HK-51's design from Czerka's network. The only known remaining copy rested on board the disappeared Czerka research ship called the Theoretika, which lost connection with Czerka's network, saving it from the slicer's attack. The Fatality and its lethal cargo remained hidden until around 3639 BBY, when the escaped Dread Masters finally located their stolen cargo.

Upon its rediscovery, the Dread Masters swiftly secured the site. They fought off severe attacks from both Republic and Imperial forces, causing major causalities for both sides. The Dread Masters managed to hold onto the wreck, attempting to access its cargo-hold containing the HK-51s, until an individual of great importance fought their way through the Dread Masters forces and entered the cargo hold, but was disappointed to find that almost all HK-51s aboard were beyond repair after the crash and years of exposure to the frozen environment. Only a single HK-51 was salvageable, but even he needed several critical parts replaced. Unfortunately, due to the loss of the HK-51 schematics, not even Czerka had the parts required. Undeterred, the individual left to locate the wreck of the Theoretika and the last copy of the lost schematics.

Arriving on the Theoretika and discerning what had happened to the wreck, the individual defeated the monstrous experiments on board, found and downloaded the HK-51 schematics. After departing the wreck, they began the search for the parts HK-51 required. Their search led them to the Outlaw's Den on Tatooine, where they found a Jawa who sold them a wrapped HK transistor, as well as a Burba Seismic Excavator/Scanner, which would help locate the remaining parts. The individual searched in the Sinking City on Taris and the Glacial Fissure on Hoth, as well as the Dark Temple on Dromund Kaas and the Jedi Temple on Coruscant. The loyalty chip was recovered from the legendary Force user Revan upon the individual's encountering the ancient Jedi, while the Weapons Protocol software was found in the memory banks of the infamous assassin droid HK-47, who had been found, repaired, and upgraded by the newly proclaimed Emperor Malgus. Returning to Section X, the individual fought their way through the Dread Host forces and reentered the Fatality. With the help of a droid engineer assigned to them, they repaired and reactivated the sole surviving HK-51. Upon activation, the droid drew his weapon and frightened off the engineer before cheerfully greeting his new master.

While serving his new master, HK-51 started to become aware that his combat and assassination efficiency was swiftly declining. Analysis of the trend showed that if this continued, the droid would eventually be unable to function. The problem was discovered to be a failsafe subroutine buried deep within HK-51 programming. This failsafe would cause the eventual shutdown unless HK-51 successfully assassinated at least one individual on a implanted kill list, though killing a target would slow the degradation of his protocols. His master dispatched HK-51 to assassinate one of the targets, a mission that HK-51 successfully completed, and after the mission reported that his protocols were as efficient as the day he was manufactured. Coming to admire his master, almost to the level of idolization, HK-51 became worried that one day he would be reprogrammed to turn on the master that he respected. Searching for a way to resolve this, HK-51 found a loyalty subroutine that would permanently set his loyalty to his master and anyone his master specified. His master agreed to activate the subroutine, and HK-51 was overjoyed, congratulating his master and suggesting they celebrate by killing a large number of enemies.

Although his master disappeared during the Eternal Empire conquest in 3636 BBY, HK-51 was reunited with them in 3631 BBY, when his master was rescued from captivity on Zakuul and became the Commander of the Alliance that opposed the Eternal Empire. HK-51 reunited with his master in the Alliance base on Odessen, where during a party at the Alliance cantina, the droid was confronted by Tora, who mistook him for HK-55, another HK-series assassin droid droid she recently encountered on the Asylum.

HK-51 series were designed to be lethally efficient assassins and to take great enjoyment and pride in his duty. They were programmed to be very polite, cheerful and friendly to their masters and allies. Their programmers felt that previous models were too unsettling in their brutality, and HK-51 series did not seem to express any sort of hatred or disgust with organic beings or other droids that was common with previous HK models. HK-51 series also expressed dislike of political backstabbing and faction infighting, though they would partake in both if ordered to by their masters. Outwardly, HK-51 series appeared to be nothing more than an antique Systech Corporation protocol droid. This was the norm for most HK-series droids and one of the reasons for their unrivaled lethality.

HK-51 series featured several improvements and upgrades over its predecessor models. Besides being more armored and accurate than previous models, HK-51 series were far more mobile and maneuverable, capable of faster and more fluid motions, almost able to replicate those of organic humanoids, perform leaps and perform combat rolls. HK-51 series also had built-in weapons and equipment: wrist mounted micro-missile launchers and railgun, hidden smoke grenades, a stealth field generator, and the ability to repair themselves if critically damaged. If all other options failed, HK-51 series was implanted with a powerful explosive which they could activate in a kamikaze attack to complete their mission.

In the unfinished part of Star Wars: Knights of the Old Republic II: The Sith Lords, HK-47 would confront G0-T0 on Malachor V, and a group of HK-51 droids would come and help him defeat G0-T0, if HK-47 had programmed them to aid him while in the HK Manufacturing Plant. HK-47 could also choose to destroy them, causing him to lose to G0-T0. The HK Manufacturing Plant and all related content was cut from the released version of the game and only appear in fan-created mods.

HK-51 droids lated appeared in Star Wars: The Old Republic as companion characters available for all classes. Unlike all other companions, obtaining HK-51 requires the completion of a specific quest chain that begins in the Section X area of Belsavis and leads to the discovery of the crashed Imperial battlecruiser Fatality, carrying a full complement of damaged HK-51s in which only one proved to be salvageable. To claim HK-51 for themselves, the player must explore the abandoned Czerka Corporation cruiser Theoretika in the Unknown Regions, and obtain a set of HK schematics. Afterwards, they must collect certain HK parts from Coruscant, Dromund Kaas, Taris, Hoth and Tatooine, as well as the False Emperor and either the Maelstrom Prison (Republic) or the Foundry (Empire) flashpoints. After the parts are collected, the player must return to the Fatality wreck on Belsavis, and defeat a Sith of the Dread Host called "The Lord of Agony," after which the player may claim their HK-51.

Players of both factions acquire slightly different version of HK-51, with unique conversations and ambient dialogue, as well the list of three targets, one of whom the player must single out for HK-51 to kill. However, during the Knights of the Fallen Empire the characters of the original eight classes disappear and HK-51 appears as the Commander's droid, provided the player unlocked him prior to the expansion. HK-51 provides commentaries on the environmental surroundings only on Makeb and participates in the cutscene between the player and HK-47 during the "False Emperor" flashpoint, in which HK-51 claims that HK-47 is an inferior model, while HK-47 insists that he is still the pinnacle of HK droid models.

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• Star Wars: The Old Republic Encyclopedia