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 and Tactics
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 converging.
 * 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.
 * Cape 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.
 * Pufferfish: This attack stays virtually identical except for the faster rate that the fireballs travel at.
 * 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. This can be reduced to 12 hits if each hit happens less than one second away from each other. 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.

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 Spikes, 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 player 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. Immediately jumping during this attack will require a small double jump then followed by the dash to allow for enough air time to not land on the fourth bat.


 * Dive: Jump high into air and avoid fire. The player can also time a down strike on Nightmare King to damage him during this attack. Dashing after hitting him is recommended to gain enough air time to not land in his fire trail.


 * Dash uppercut: If there is enough room always dash away from him, then attack or Great Slash him when he he finishes his dash and before his leap into the air. If there is not enough room in the arena to dash away from him, the player will have to pause and wait instead of immediately trying to Shade Cloak dash through him as he will skitter away and leave the player in a position where damage in inevitable. The timing can be tricky when trying to dash through him, but doing a short hop is generally enough time to allow Nightmare King to not skitter away.


 * 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 this attack try and hit him a two times while walking beneath him 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. Abyss Shriek is recommended if possible right after the fourth pillar has started.


 * Pufferfish: It is best to stay close to the wall but not to stick to it as the player needs to move back a little to dodge a fireball. The best way to dodge this without taking damage is to make small jumps so the player doesn't accidentally hit the fireballs flying over their head. Another way is to climb the wall and jump onto to Grimm and use down strikes to bounce on him. Though to do this the player must take damage to take advantage of the invincibility frames or be really quick as soon as he starts the attack.

Healing : Without any healing charms, the player can heal: once right after the last flame pillar begins to form, once by jumping over one firebat and then immediately healing (the next low firebat will have had its flight pattern altered by its homing ability and will fly over the Knight's head, allowing time to heal) or right as the Cape Spikes attack starts though the timing on this combined with the positioning aspect makes it more tricky. Three heals are also possible during the stagger as long as the player starts healing as soon as the stagger starts and is prepared to move as soon after the third heal.

Shape of Unn can be used to heal under the Firebats attack as long as the player immediately starts healing once the attack starts (as the animation to transition into a slug takes enough time that healing too late will cause the player to be hit instead). in addition healing in cape spikes is made easier with Shape of Unn and healing is possible during the Flame pillars attack with the same conditions as healing during the Firebats attack.

The player can also heal by dashing away from his uppercut usually putting them in a safe position and immediately healing using Quick Focus.

Charms : Dreamshield blocks the Firebats attack.

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

Nailmaster's Glory, Fragile Strength / Unbreakable Strength, and Mark of Pride can be very helpful for reaching and damaging him.

Note that Grimmchild was added as a requirement to start this fight as of the Lifeblood update.

Trivia

 * Other than The Radiance, Nightmare King Grimm is the only other boss that has a fullscreen title card at the beginning of the fight.


 * Nightmare King Grimm looks nearly identical to his physical world counterpart Grimm (say for the red colour scheme), however Nightmare King's horns are slightly longer.


 * An alternative way to predict Nightmare King Grimm's health is to observe the background as it changes per each Flame Spawner attack. At 75% health the heart beats faster, at 50% the hearts eyes glow red and at 25% the heart beats ever so faster the glow is more intense.


 * 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 Grimm.

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