For the NPCs that assign creatures, see Slayer Master. For the title, see Slayer Master (title).
Slayer Master is an achievement that requires the player to complete wave twenty of the platinum difficulty of Rush of Blood and earning the title Slayer Master [Name]. Twenty waves need to be cleared, reaching wave 20 does not suffice.
The title can be unlocked on either a reward or trial run. If any of the previous titles (the Abyssal, the Dark, and the Beast) weren't unlocked yet, then those will be unlocked at the same time.