Grimm

After the Grimm Troupe has been summoned, Grimm can be found inside the large building at the center of their setup to the left of Dirtmouth. Inside, after talking to him, he gives the Knight the Grimmchild charm. After collecting three flames, he upgrades the charm to the next level, and after collecting three more, the Knight must fight Grimm.

Behaviour and Tactics
Before the battle, Grimm will bow. If the Knight hits him while bowing, Grimm will screech at the Knight and will counter attack with the Pufferfish Attack.

If Grimm teleports on the ground too close to the Knight, he will Skitter away on all fours before resuming his attack. It is rare but possible that Grimm will begin a telegraph for one type of ground attack, Skitter away, and then do a different ground attack.


 * Fire Bats: Grimm will open his cloak, sending three Fire Bats at the Knight. The Fire Bats start at the level of the Knight's head and poorly track the Knight's movement. Grimm remains stationary during this attack. Each Fire Bat's entrance is announced with a gout of flame coming from Grimm and reaching the top of the arena. If the Knight gets within arm's length of Grimm before he sends out all three Fire Bats, he will teleport to another part of the arena and send one last Fire Bat. This Fire Bat cannot be interrupted. This means that it's possible for Grimm to send out only one Fire Bat.


 * Dive Dash: Grimm will wrap himself in his cloak until he resembles a drill and dive towards the Knight. Then he lands and follows up with a dash towards the Knight's direction using his arm wrapped in his cloak like a lance. Grimm tracks where the Knight was near the start of the dive and the dive can traverse the entire arena.


 * Dash Uppercut: Grimm will dash across the arena with a large swipe directly before launching into the air with an uppercut. At the apex of the uppercut, he disappears and rains five fireballs on the arena. The five fireballs rain down on the arena in set parabolas.


 * Cloak Spikes: Grimm will send his cloak into the ground, causing large twisted cloak spikes to jut up from the floor. The spikes reach to the top of the arena but there are large enough gaps between them to stay safe. The spikes are not dangerous until they are at their tallest. The spikes persist for about half a second.


 * Pufferfish: Grimm surrounds himself with his cloak and puffs out and spews tiny fireballs all over the arena. This attack occurs when Grimm's health reaches 75%, 50%, or 25%. This attack always occurs from the center of the arena. The fireballs travel in five general directions. Fireballs tend to come at the Knight in sets of two, resulting in essentially a jumping puzzle. Grimm will stay in this form for nearly nine seconds. While he's in this mode, Grimm is immune to Nail damage but still takes damage from everything else.

Grimm is one of the faster bosses in the game, and his health scales with nail upgrades, making him a tough foe regardless of experience or progress.

Grimm's stagger is unique only to him and his dream variant. When Grimm is staggered, he disperses into a swarm of tiny bats resembling the Grimmchild that fly haphazardly around the arena for about three and a half seconds. The Knight takes no damage from the swarm but can still hurt the one Grimmchild with red eyes during this time (for a maximum of 50 damage regardless). Grimm's stagger is over when the swarm all collect and reform into Grimm.

Because Grimm moves by way of teleportation and rarely stays in one place for long, from an offensive perspective, it's important to deal as much damage to Grimm in the shortest time possible.

Each spell has its use against Grimm. Vengeful Spirit is useful to use during the Cloak Spikes, Pufferfish Attacks and Fire Bats Attack. Desolate Dive is a great way to dodge the Dive Dash Attack. It's best to use it almost as soon as the player sees Grimm preparing the Dive Dash Attack because Grimm will land in the middle of it. Howling Wraiths can be useful during a Pufferfish Attack right before the fireballs start to spawn and during a Cloak Spikes Attack since Grimm is taller than the Knight.

The Nail Arts Great Slash and Dash Slash are also useful, providing 2.5x Nail damage and good range for damaging Grimm and staying safe (especially during Cloak Spikes, Dash Uppercut, and Fire Bats Attacks).

Use the Shade Cloak to dash through both of his Dash attacks (Dive Dash and Dash Uppercut). The player can also damage Grimm this way if they have Sharp Shadow.

Standing directly next to where Grimm explodes into fire when he uses Dash uppercut is a good way to avoid all 5 projectiles as they spread in a cone pattern, this also gives healing opportunity especially with Quick Focus.

During the Cloak Spikes attack, The player can use the time to heal. This is easier to do with Quick Focus.

Shape of Unn is a fantastic way to heal against Grimm. The charm lowers the Knight's hitbox so players can heal safely during a Fire Bats Attack. If combined with Quick Focus, the player can run and heal from Grimm during a Dash Uppercut Attack too. It can also be used to heal and position at the same time during the Cloak Spikes attack.

If the Knight stays near the the right/left side of the screen and light tap jumps, they can mostly avoid the flames during Grimm's Pufferfish Attack.

Double jump and dash over the Fire Bats, putting the Knight in range to attack.

Trivia

 * At the beginning of the battle Grimm will bow, if he is attacked before he can finish the bow, he will screech and immediately start his Pufferfish attack.
 * When fought in Godhome, Grimm bows to The Godseeker instead of bowing to The Knight.


 * The Grimmchild charm is required every time to start the battle.


 * The fight with Grimm is accompanied by audio cues for some events:
 * When Grimm performs the Pufferfish attack, the theme gets more dramatic by adding chorus to background.
 * The Grimmkin audience watching give ovation to Grimm every time he performs the Pufferfish attack or reforms after stagger.
 * When Grimm reforms after stagger, he is accompanied with a drumroll.


 * There is a secret area in Grimm's tent above the arena where the empty masks of the audience can be found. It can be accessed by wall climbing through a false ceiling to the right of the arena, second arch.
 * When using a spell in this secret room some sort of barrier appears which may possibly be related to a Seal of Binding.


 * There is some speculation as to what species of bug Grimm is supposed to be:
 * It is possible that he is a bat due to the fact that he sleeps upside down and generally resembles a vampire.
 * In a response in one of their Hollow Knight streams Team Cherry has stated:
 * Can you confirm Grimm was inspired by Dracula from Castlevania?

William: “He’s like a mix of a lot of things.” Ari: “Yeah, he has this kind of circus master quality, and-” ''William: “Halloween-y.. He’s spooky.”''
 * Ari: “I don’t think- I mean Grimm obviously has a lot of Dracula-like qualities. But he also has other elements.”


 * A red barrier will appear over Grimm when trying to enter the Nightmare King Grimm fight without Grimmchild equipped.


 * Even in Godhome, it is not possible to fight him with Grimmchild. If equipped, it will not be present for the duration of the fight. The same is true for Nightmare King Grimm.

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