HK-50 (Hunter-Killer) was a model of assassin droid, commissioned by Revan, as a newer version of the HK-47.  The droid is sourced to the (sadly now “Legends”) Knights of the Old Republic II video game.  Apparently there was some content that was cut before the final release that involved HK-47 finding the factory, and then working to convince the HK-50’s to join him and his allies.  Failing in this, HK-47 then turned to the unfinished HK-51’s in the factory.  The storyline would have had the HK-51’s arrive during a some final battle and help save the day.  Sounds like a lot of fun.  I wonder if this may have had some influence on the arc of L3-37 in Solo.

What you need to build HK-50:

BD37 Clone Commander Deviss

BD51 Luke Skywalker

BD47 Commander Bacara

BD49 AT-AT Driver

BD54 Zuckuss

BD55 Snowtrooper

BD48 Snowtrooper (Concept)

BD52 R2-X2

BD45 Plo Koon

BD50 Wing Guard (SGT Edian)

BD43 Agen Kolar

BD53 Willrow Hood