The hidden appearance for Assassination comes from the Cypher of Broken Bone which dropped from rare spawns when the Underbelly is in PvE mode in beta.
So far on live I've got 45 kills on the rares that have the potential to drop it and no luck. Wondering if anyone else has found it from them yet or if it may have possibly been moved. Probably just bad rng for me but if anyone can confirm that would be great.