This achievement is for banishing the Grimm Troupe and requires several steps to complete.
First, head to this location behind a breakable wall in Howling Cliffs:
Use your Dream Nail on a large dead bug at the end of the cave path:
Then go back a screen and attack a fireplace several times to summon the Grimm troupe to Dirtmouth. You'll see their tents to the left of the bench. Go inside the larger tent and talk to Troupe Master Grimm to receive the Grimmchild charm. This charm summons a familiar when equipped which can be upgraded by seeking out three flames marked on your map which summon an enemy as you approach. Kill the enemy at each of these flames, then return to the Troupe Master and he will upgrade your familiar and mark three more flames on your map. Repeat this process and when you return to the Troupe Master, you will have to fight him. Defeat him for an achievement:
He will then mark four flames on your map, one of which being in the Distant Village area:
Enter the hut indicated by the flame to meet an NPC named Brumm who will ask you to help him put an end to the Troupe:
Meet him back in Howling Cliffs where you initially summoned the Troupe and he will ask you to destroy the flame. Do so, then return to Dirtmouth and talk to the new NPC with an accordion near the bench:
He will give you the Carefree Melody charm and the achievement will unlock.
Note that this achievement is in conflict with the Ritual achievement (where you fight Nightmare King Grimm) and you can not earn both on a single save file.