You need to complete the three tasks given by three NPC.
1. He's hidden in the Death Tavern, once you're in the place where you'd press up to enter the portal room, just press toad charge up, and there's a secret room, he'll ask you to kill rats in the sewers, just kill them until it's completed, then return.
2. In the Frozen Plateau Dispatch Station, there's an NPC out, talk to him, just kill fifty monsters in that area, and deliver it.
3. There's a girl in the sewers (Underground Waterway) called Dusky, she'll ask for a doll, the doll is obtainable going through a secret way after killing Ghost Bride, just check the map on steam guide and you'll find her and the way back to the doll, deliver the doll back.
And that's not all, you need to learn the 3 hidden skills (perks) they teach you, and once you learn them, the achievement will unlock.