|“||Through dream I travel, at lantern's call, to consume the flames of a kingdom's fall.||„|
|~ Grimm's riddle in the Hunter's Journal.|
Grimm, also known as the Nightmare King, is a supporting antagonist in Hollow Knight, introduced in "The Grimm Troupe" DLC. He is the villainous leader of the mysterious Grimm Troupe, a group of performers who seek to collect the flames of the fallen kingdom of Hallownest in order to prolong the Nightmare's Heart, a process the Troupe calls "The Ritual", and manipulates the Knight into doing his bidding.
Grimm first appears when the Troupe is summoned to Hallownest by the Knight lighting the Nightmare Lantern in a hidden area in the Howling Cliffs, after which they set up camp in Dirtmouth. Inside the main tent is Grimm, who greets the Knight and explains that the Grimmkin now travel across Hallownest to collect the "flame in dream". He gives the Knight the Grimmchild charm, and encourages them to seek and defeat the three Grimmkin who have gathered flames in order to upgrade the Grimmchild.
After defeating the three Grimmkin and collecting their flames, Grimm appears when the Knight and the Grimmchild return to his tent and uses the flames to upgrade Grimmchild into a deadly companion. He then sends the Knight out to find three more flames to further grow the Grimmchild so he can begin the performance. Following the next three flames, Grimm leads the Grimmchild into his body and challenges the Knight in a duel. After defeat, Grimm congrats the Knight and mentions how they have potential in prolonging the Nightmare Heart. He awards them with a charm notch and returns the Grimmchild, now at its third form, and encourages the Knight to seek out the last three flames so the Ritual could begin.
Following Grimm's boss fight, the player is given two options to complete Grimm's quest: first is to collect the remaining three flames and return to Grimm, who now sleeps in the back room of his tent. Dream Nailing him takes the Knight to Grimm's dream in the nightmare realm, where they must fight against Nightmare King Grimm to complete the performance and finish the Ritual. Defeating him there will transfer the Nightmare's Heart from Grimm to the Grimmchild, granting the Knight the fourth and final form of the Grimmchild. Defeating Grimm causes the Grimm Troupe to vanish, although Divine will stay behind and upgrade the fragile charms.
However, talking with Brumm in the Deepnest before collecting all three flames and following him to Howling Cliffs to quench the flame of the Nightmare Lantern will cancel the Ritual and banish the Troupe. The Nightmare Lantern is the anchor through which the Grimm Troupe can manifest in the waking world, and destroying this anchor banishes the Troupe back to the nightmare realm.
In his regular appearance, Grimm is a slender dark being covered by a multicolored cloak, with a gray exterior and a red interior. His face is pale with red eyes, and underneath his cloak, his body is scaled and purple. As the Nightmare King, Grimm's body turns entirely red, with his cloak having a maroon color. His face remains pale and his limbs remain black.
- Grimm (especially his Nightmare King form) is based on vampires, most notably Dracula.
- At the beginning of the battle, if Grimm is attack while he is bowing, he lets out a banshee-like screech and retaliates with his Flame Spawner attack, which he normally only uses when his health reaches 75%, 50%, or 25%.
- The Grimmchild charm is required for both battles. The Grimmchild itself, however, cannot participate in any of Grimm's battles, even in Godhome.
- The audience in the background applauds every time Grimm uses his Fire Spawner attack and reforms after being staggered.
- At the beginning of the fight, Grimm will always bow towards the Knight unless in Godhome, where he will bow towards the Godseeker audience.
- Grimm's health is determined by the Knight's Nail upgrades, gradually increasing depending on the upgrade level. This also applies to the Grimmkin, the Dream Warriors, and Hive Knight.
- Nightmare King Grimm is tied with the White Defender as having the second-highest health of any boss in the game, behind The Radiance.
- Along with the Radiance, Nightmare King Grimm has a fullscreen title card.