Based off of Knights of the Old Republic's HK-47 (who is also in SWTOR), HK-51 is a more advanced droid assassin model that prizes "efficiency over brutality." While remarkably effective at killing, the HK-51s were all but destroyed. Only a single crashed ship carrying the remainder of their kind is reported missing on Belsavis. BioWare also mentions another starship that may have design blueprints for the droid, but it too is lost.
The droid assassin will be obtainable in a new area on Belsavis that also comes with the update, although free players will need to purchase this content to gain access to the companion.