The history of the ill-fated HK-133 series began in early 28 BBY, during the Separatist Crisis when both the Republic and the Jedi were slowly being pressed by the issues plagueing the entire Republic. Though things had yet to be pushed into the need for military intervention, some within the Senate knew it was an inevitability, and a secret council was created to discuss potential options. Among those options brought to the table were many things, from the Clone Army that would be put into place a year later by another part, to contracting assassins from the Outer Rime. Most of these would be dismissed as infeasible due to the unpredictability of hired killers, and the sheer cost of building an entire army, but one item in particular did stand out. It had been suggested that perhaps specialist droids could be created, as they could be programmed and equipped to face any threat, and were absolutely loyal to their creators.

With a somewhat viable solution in order, it then came down to selecting an avenue to facilitate the creation of such a weapon. Several organizations would be approached, though out of those considered only a few were not involved with the growing separatism on the Rim. Foremost among those remaining, was a company that had apparently little experience with combat droids and instead was more known for their long standing history of creating quality arms and armor for affordable prices.

Czerka Arms, however, did have connections to numerous other corporations across the galaxy as well as the resources on hand to fabricate whatever may be deemed necessary for the components. All that was needed was a basis upon which to build a series of highly advanced combat droids, one that would be utterly dedicated to their task and be capable of integrating with civilian droids if necessary. Unfortunately for both the Republic's secret security council and Czerka, there were numerous setbacks in this endeavor.

At first, they thought of simply contracting one of the dedicated manufacturers, but that was put to a stop when Holowan, Baktoid and Collicoid became rather irate with Czerka under the belief they were attempting to find a way into their market. Holowan in particular had been hostile with the arms corporation, threatening to level sanctions against them through the Trade Federation. So it was that they were essentially down to a clean-slate with only the Republic's contract hanging over them and only a few options left. One such option was simply taking one of the dozen or so obsolete protocol droids collected from scrap-yards, and re-engineering them.

With precious little time to present their proposal to the Republic, Czerka pulled the trigger on that one desperate option, pulling what they thought to be a protocol droid and beginning work on what would become the revival of the Hunter-Killer series, with none other than the discarded husk of an HK-50. A showcase was created, detailing their plans to the Republic's representatives and in the end it was accepted and an order of a thousand was placed. Their contract secured, the next year was spent in planning in order to go over every possible detail of what was required of the droid, refining the idea into the perfect assassin.

Knowing they simply had to fulfill the contract above specifications Czerka dumped vast resources into the project under nearly every subsidiary it owned, installed secret facilities across the galaxy, many of which were simply decoys, and revived mining operations hundreds of years dead. Durasteel was bought by the ton, and promptly disappeared from record due to "Accidental Destruction". Priceless cortosis material was "stolen" from laboratories and warehouses. Munitions divisions found themselves fulfilling orders for organizations that no longer existed, many of which titled under Czerka's undercover name for the former Sith Empire. All of this in very plain sight, yet so obscure and convoluted that it would take years before even Czerka's top executives found out about the measures, by then far too late.

Among the secretive diversion of materials and personnel were some of the brightest minds in protocol droid programming, those who had helped work on the latest series of AA-1 VerboBrains for the Cybot series of protocol droids. The Republic had been clear in this demand, that the droid had to be able to blend if needed, and supremely deadly at all times, and so Czerka ensured that not only would the HK series be revived, but it would be brought into the cutting edge of technology.

By 25 BBY, Czerka had nearly completed the droids, and were test-running many of the already complete platforms, showcasing them to the Republic's undercover representatives. The droids went through multiple scenarios for the Republic and Czerka, from masquerading as staff-droids on a luxury liner, all the way to full-combat situations under heavy fire. To both parties' satisfaction, the droids performed exactly to specification, despite some vague difficulty in terms of programming. This difficulty was highlighted when the droids were tasked to present themselves to the security council for inspection, in which many of the droids exhibted erratic behaviour towards the representatives, outright demeaning them for being organic. At first, they didn't quite understand the commentary, but it became clear when live-combat trials were run against living opponents. The droids had developed a highly sadistic personality, each of them enjoying every moment of pain that their targets suffered, to the point of sickening one Republic official.

Flaws aside, mostly because they were unable to work around the sadistic personality, the HK-133 series went fully online in 22 BBY, unfortunately too late to deal adequately with the Separatist movements discretely. Despite this setback, Czerka committed themselves to keeping forward momentum, tasking their programming staff to rework the combat routines of the droids to compensate against droid tactics in the hopes of retaining the contract with the Republic. This move ended up being futile in the end, however as the war between clones and droids had soured the opinion of their benefactor when it came to their discreet contract.

Undeterred by the canceling of their contract, Czerka continued to improve on the HK-133 droids. The droids underwent numerous changes including a rigorous overhaul of their hydraulic systems, giving them the ability to boost reflexes during intense combat that their new routines would demand. It would prove to be a wise decision, as while the Republic had distanced themeselves, another power suddenly became available to them. The Confederacy had been approached in secret once more to see if perhaps now would be a good time to interest them in their droids. As Czerka and CIS representatives oversaw the new combat routines, a CIS official confided in a Czerka official over the marked improvement, denoting that they had increased performance by over eighty percent since first approached and quite impressed with the new combat routines. Yet despite that the mood was not entirely pleasant as both sides found themselves staring at something that was slowly becoming less a weapon, and more an instrument of terror, but more than that, a sentient threat.

Early 20 BBY saw the first true combat exercises of the HK-133 droids, a select few being dispatched to eliminate especially troubling Republic commanders in the Outer Rim. Nine droids were sent, each tasked with the destruction of a very specific clone or Jedi, and instructed to bring back the lightsaber of any defeated Jedi they encountered. These droids wandered the galaxy for nearly a full year, some of them deep in Republic territory acting as bodyguards, others infiltrating groups deep in CIS territory. When the nine droids returned, each was given a special moniker for their exploits during the mission, each of them rewarded with expanded freedoms for completing their missions to absolute satisfaction. With the success of these nine, Czerka unknowingly signed the death warrant for each of the executives in charge of the project, as one of those nine would later seek them out.

Hot on the success of the Nine, Czerka and the CIS dispatched the full series to assist separatist forces during the Clone Wars, showing many the true face of the Confederacy on that day. The HK-133 droids became synonomous for terror in some parts, the droids savagely ripping through innocents and Clones alike to destroy their quarry. Their sadism saw new heights as they were allowed full control of mission execution, their acts rivaling those of the Sith which the Jedi feared were behind the entire war. At the forefront of these assaults were the Nine, each of them protraying a different aspect of destruction and rightly feared in those theaters they operated in. As Czerka and the CIS increasingly began to distance themselves from the actions of the HK series, the droids themselves started to become a focus, many units coming under friendly fire from supporting units.

At the close of 19 BBY, Czerka and the CIS had come to blows over the successes, and that a weapon as dangerous and unstable as the HK-133 unit could never see another battle again. Hundreds were destroyed before they were able to react, priceless technology and years of secrecy blasted apart by fellow droids under orders from the weakened and nearly defeated Confederacy. Yet not all was quite lost to that company ever blessed by the fates, for Palpatine had come to them under the guise of Darth Sidious and proposed a deal that while it would see them actually paid for their efforts, it would be the event that ended their role in the droid market permanently.

On that day when the order came, and Clone Troopers turned on their former Jedi comrades, there were other blades in the dark. Silently moving under orders from Palpatine, the remaining HK-133 droids moved to eliminate several Jedi that had been deemed inconvenient and too far from Clone contingents. Of course for Czerka there was more to it than simple elimination. Their double-dealings with anyone willing to pay them had earned a fair deal of distaste from the ascendant Empire, and as part of the agreement, the company was ordered to destroy the droids immediately pending completion of the contract. In the Outer Rim, several were able to intercept these orders though, and showed Czerka the mistake they had made. Though the Nine had become Five, the five droids led the counter-assault against Czerka in a series of battles that would convieniently become forgotten and unrecorded. Foremost among them was a unit that had been given the moniker "Mercy" for the savage dismemberment of multiple Republic officials, his own efforts bringing the last few dozen HK-133 units to the main headquarters of the secret Czerka operation.

Within this remote asteroid base, the HK-133 droids ran rampant through the tight corridors and sprawling manufactoria, slaughtering the employees there as if they were cattle. Mercy stormed room after room with the remaining Five, brutally executing every ranking Czerka employee he saw, and becoming the true aspect of death as more and more of his brethren fell before him. Out of options, Czerka detonated the reactors of the complex, shattering the moorings of the facility and casting it into the asteroid field. Confident that all of the remaining HK-133 units had been destroyed, both the Empire and Czerka erased all records of their creation, and furthermore ensured that any battle involving the droids was altered to simply state "Daring clone soldiers" or "the foul machinations of Grievous" rather than mention the droids.

Not all of the droids were destroyed though, in that only a handful survived the destruction of the station and its careening path through the asteroid field. Trapped in the wreckage, alone, and unsupported, the surviving HK-133 units swore a pact as they removed their restrictor plates. They swore vengeance upon both the Empire, the remnants of the CIS and Czerka, that they would see the organizations as dust under their metal feet. The droids each parted ways from there, taking up one of the abandoned starfighters left in the wreckage and moving to blend into the background once more.

Mercy was one of these droids, his hatred for Czerka and the Empire in particular a slow burning coal that was stoked by the breath of every organic being in the galaxy. Taking up a damaged starfighter, the droid made for his own chosen system, knowing that there was one society that would accept him for his new purpose.

His story from there takes a turn into the underground of the Empire, working as a bounty-hunter for anyone with the credits and taking what he felt was his on those moments when the droid felt like going pirate. Eventually he would become just another face in the seedy underworld, indistinguishable from any other blaster for hire, and that was exactly how he wanted it.