This is the optional final step in the Grimm Troupe questline, with the other option being to talk to an NPC and banish the troupe, earning the Banishment achievement
. You cannot get both Ritual and Banishment on the same save file.
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 and talk to Troupe Master Grimm to receive the Grimmchild charm. Seek out and kill three enemies that are marked with flames on your map while having the charm equipped, then return to Grimm. The familiar granted by the charm will be upgraded and you'll be tasked with defeating three more enemies marked by three new flames on the map. Do so and return to Grimm once more, which will start a battle with him. Defeat Troupe Master Grimm to unlock an achievement:
The familiar will be upgraded and three more flames will appear on the map (four, actually, but you can ignore the flame in Distant Village area as that marks the NPC that leads to the Banishment achievement)). After defeating these three enemies, return to the Grimm tent in Dirtmouth and you will find Troupe Master Grimm hanging upside down asleep. Hit him with the Dreamnail (hold
to use) to start the battle with Nightmare King Grimm, a more difficult version of Troupe Leader Grimm with similar attacks.
He has five basic attacks that you'll need to know how to avoid as everything does two masks of damage to you:
Diving dash: Just jump over him to avoid taking damage
Charge to uppercut: dash away to avoid him, then avoid the falling projectiles
Cape Spikes: get in between two to avoid damage
Firebats: jump and dash towards him and attack when within range
Fire pillars: attack him a few times while avoiding the fire
At certain health increments, he will take position in the center of the screen and release a torrent of fireballs that must be avoided. Also, after taking enough damage he will stagger (turn into bats and then reappear), which is really the only time you can heal.
I opted to instead maximize health with charms and never bothered healing.
After defeating Nightmare King Grimm, your familiar will receive the final upgrade and this achievement will unlock.
Here is my successful battle for reference: