Nightmare King Grimm

After defeating Grimm, he upgrades the Grimmchild, allowing The Knight to collect another four flames. After collecting at least three of them, Grimm will be asleep at the back of the large building in Dirtmouth. After using the Dream Nail on him to enter his nightmare, the Knight must fight him, and complete the ritual.

Behaviour
The fight functions similarly to the Dream Bosses. The base fight remains similar, although it is much faster paced, and most attacks gain new properties.
 * Firebats: Grimm opens his cape and fires 4 tracking firebats in a high, low, high, low pattern. Each firebat also spawns a fire pillar on top of Grimm.
 * Firebat Teleport: Getting into melee range before the 3rd firebat causes Grimm to teleport to the other side of the arena and fire both remaining bats at once. These bats start high and low before convering.
 * Dive Dash: This now leaves a trail of fire behind Grimm that lasts a few seconds.
 * Dash Uppercut: Both the fire and the attack are faster.
 * Cloak Spikes: Grimm no longer appears during this attack, and is thus unable to be damaged.
 * Flame Pillar: Grimm floats above the ground, and spawns four fire pillars one after the other below The Knight, they fire after a short delay and leave a lingering patch of fire for a few seconds.
 * Flame Spawner: No longer a random attack, instead Grimm does this once each at 75/50/25% (1200,800,400) health.
 * Evade: Occasionally, if you get close to Grimm before his grounded attacks begin, he will scurry away before attacking.
 * Stagger: Every 13 hits, Grimm will explode into a horde of bats. Grimm can be damaged in this state, taking the form of the bat with eyes. However, he can only take a maximum of 50 damage per stagger.

Tactics
Similarly to the first battle, it's all about reading what attack is coming and knowing how to dodge and/or retaliate. The only catch is that he is much faster this time, the windows for attacking him are smaller (or in case of Cape, completely removed), and Nightmare King deals two masks of damage opposed to Grimm's one mask of damage. Shade Cloak can be invaluable for this battle, as The Knight can dash through most of Nightmare King Grimm's attacks if timed properly.
 * Firebats: Wait for the third Bat (to avoid Teleport) and jump-dash between them and try to attack him with normal slashes or hit with Dash Slash.


 * Dive: Jump high into air and avoid fire (don't try to attack during this move).


 * Dash uppercut: First dash away from him, then attack or Great Slash him when he dashes close but before he jumps into air.
 * Flame pillar: Keep slowly moving in one direction so pillars will erupt behind the Knight, try avoiding being stuck in corner of arena (if necessary Shadow Dash through Pillar). During that movement try to pass and hit him a few times or wait till the 4th pillar erupts and try to get as many hits in as you can (or one Great Slash) before he warps away.

Healing
Without charms or Deep Focus only during Stagger, with Shape of Unn and/or Quick Focus during Pillars (this can be hard to pull off), with Quick Focus during Cape or after 4th Pillar.

Charms
Dreamshield blocks the Firebats and Firespawner attacks and sometimes Nightmare King Grimm himself as he passes next to the Knight

Sharp Shadow helps with attacking during the Firebats attack, since The Knight's dash is slightly longer and it's easier to hit Nightmare King Grimm from farther distances

For Nail-focused strategies: Nailmaster's Glory, Fragile Strength / Unbreakable Strength, and Mark of Pride can be very helpful

For spell builds: Shaman Stone + Flukenest is quite powerful, though this is relatively hard to use due to the need to be close to hit. However, especially when combined with Shade Soul, this can deal a lot of damage.

Joni's Blessing + Fragile/Unbreakable Heart + Lifeblood Heart will dramatically increase your chances of survival. You have a very, very small window to heal and attack, so you might as well maximize your HP to give yourself more room for error.

Note that Grimmchild is not needed to start this fight and it does nothing during the fight so it is recommended to unequip it.

Trivia

 * Except for The Radiance, Nightmare King Grimm has the highest health of any boss in the game, tied with White Defender.
 * Along with the Radiance, Nightmare Knig Grimm is the only boss that has a fullscreen title card.
 * His design is based on Dracula.

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