The HK-77 assassin droid was considered the pinnacle of the HK-series assassin droid line. It was based on the HK-47, HK-58 and HK-65 models. It was slightly shorter than the other models, and had a narrow head with optical sensors that extended from each side of it. Unlike the earlier models, the HK-77 had blaster rifles integrated into its arms for quick and easy access.
It was considered extremely aggressive and preferred overt action to stealth. It would often reveal itself to a target and give it an opportunity to escape or attack first.
It cost 19,865 credits new. A popular modification was to convert the HK-77 into a firefighting droid by replacing the blaster rifles with fire extinguishers and coating the armor with a heat-resistant material.
The HK-77 assault droid was designed by Separatist engineers after finding and attempting to reverse engineer HK-47. Unlike their predecessor, these droids are made for more direct combat. HK-77s were designed to serve as bodyguards or henchmen to larger droids. At least one HK-77 unit found its way into a gang led by HK-47 on Mustafar, surviving into the Galactic Civil War.