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 as compared to Troupe Master Grimm's. A note about this fight: The Nightmare King will skitter away from the Knight on all fours before commencing an attack if the Knight is too close to him. However, "too close" is subjective. If he's preparing to do a Fire Bats Attack, "too close" can mean anywhere within about 4 knight's heights. If NKG is preparing to unleash a Dash Uppercut, "too close" means being within 1 knight's height within .5 to nearly 1 second from appearing. It is rare but possible that NKG will begin a telegraph for one type of ground attack, skitter away, and change to his other ground attack.


 * Fire Bats: Nightmare King Grimm will open his cloak, sending out 4 Fire Bats towards the Knight. These Fire Bats poorly track the Knight's movement. NKG remains in place during this attack. Each Fire Bat's entrance is announced with a gout of flame reaching the top of the arena that persists for nearly half a second. Each Fire Bat spawns from the same location (about the height of the Knight's head) and travel in a pattern: first and third bats go high, second and fourth go low. There is a wide gap between the rows of Fire Bats. If the Knight gets closer than NKG's outstretched hand before he sends out all 4 Fire Bats, he will teleport to another part of the arena and send 2 more Fire Bats. These Fire Bats travel simultaneously one low, one high. They meet up on the other side of the arena. It's possible for NKG to send out only 2 Fire Bats for the entirety of this attack.
 * Dive Dash: Nightmare King Grimm will wrap himself in his cloak until he's pointy and fiery and then he dives towards the Knight. He lands and follows up with a dash towards the Knight using his arm wrapped in his cloak like a fiery lance. NKG's movements during this attack are followed by large, intense flame. The dive tracks to where the Knight was near the start of it and it can traverse the entire arena. The dash covers about 40% of the arena.
 * Dash Uppercut: Nightmare King Grimm will dash across the arena with a large fiery swipe directly before launching into the air with an uppercut. At the apex of the uppercut, he disappears and rains 6 fireballs on the arena. The dash covers about 25% of the arena and includes the Nightmare King making a huge downward fiery swipe with his arm and cloak. The uppercut continues in NKG's original direction, taking him to the top of the arena. There he disappears in a burst of 6 fireballs that rain down on the arena in a set pattern.
 * Cloak Spikes: Cloak spikes 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 dangerous .75 seconds after appearing (just before they're done forming). The spikes persist for about .5 seconds. Nightmare King Grimm is not present during this attack (unlike Troupe Master Grimm).
 * Flame Pillar: Nightmare King Grimm summons 4 large pillars of flame. He remains stationary in the air (just lower than the start of a Dive Dash attack) throughout this attack. The pillars are summoned one after another every .75 seconds. Each pillar reaches the top of the arena and starts forming where the Knight was. As a pillar begins shrinking away, a wide, shorter flame appears and remains on the ground for 1 second.
 * Pufferfish: Nightmare King Grimm surrounds himself with his cloak, puffs out, and spews small fireballs all over the arena. This attack occurs when NKG's health reaches 75%, 50%, and 25%. This attack always occurs from near 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 (though this whole attack is generally considered a "bullet hell"). The Nightmare King will stay in his pufferfish form for nearly 9 seconds. While he's a pufferfish, NKG is immune to Nail damage but still takes damage from other sources. The fireballs are faster than Troupe Master Grimm's.

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 Cloak 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.

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 (save 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.


 * 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 Grimm.

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