Nightmare King Grimm

"Twisted spectre of scarlet flame. "The expanse of dream in past was split, One realm now must stay apart, Darkest reaches, beating red, Terror of sleep. The Nightmare's Heart.""

- Seer

Nightmare King Grimm is the optional final boss of The Grimm Troupe in Hollow Knight. It is the Dream form of Troupe Master Grimm.

Behaviour and Tactics
To start the fight with Nightmare King Grimm, it is required to have Grimmchild equipped.

Much like most other Dream Boss battles, the base fight with Nightmare King Grimm remains similar, although it is much faster paced, and most attacks gain new properties:
 * Fire Bats: Nightmare King Grimm sends out 4 Fire Bats instead of 3. They travel in a pattern: first and third bats go high, second and fourth go low, with a wide gap in the middle. If the Knight gets within his arm's length before he ends the attack, Nightmare King Grimm will teleport and send 2 more Fire Bats, one high and one low, simultaneously.
 * Dive Dash: During both dive and dash, Nightmare King Grimm is followed by a flame tail that deals damage. The Flame Trail generated by the dive disappears after he lands on the ground. Then he stands up after the dash before disappear again.
 * Dash Uppercut: The pause before Nightmare King Grimm launches himself into the air is shorter. Nightmare King Grimm bursts into six fireballs instead of five.
 * Cloak Spikes: The spikes become dangerous in a shorter amount of time. Nightmare King Grimm is not present during the attack.
 * Flame Pillar: Nightmare King Grimm summons 4 large pillars of flame. He remains stationary in the air throughout the attack. The pillars are summoned one after another. Each pillar starts forming where the Knight was and reaches the top of the arena. As a pillar begins to blench away, a wide, shorter flame appears and remains on the ground for 1 second.
 * Pufferfish: The Fireballs travel at a considerably faster speed.
 * Teleport: Nightmare King Grimm teleport to a random place in the arena to perform an attack.
 * Skitter: Nightmare King Grimm will skitter away from the Knight before starting an attack if they are too close to him. Nightmare King Grimm may begin to telegraph for one type of ground attack, skitter away, and change to another ground attack

Similarly to the first battle, it is beneficial to read what attack is coming and know 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 Knight can dash through most of Nightmare King Grimm's attacks if timed properly.
 * Fire Bats: Short jump over the 2nd Fire Bat and then dash while in the air to avoid the 3rd and 4th Fire Bats (this is safest with Shade Cloak). Knights can follow this dash up with Descending Dark, Shade Soul, any Nail Art, or regular Nail strikes. If the second Fire Bat is jumped over, staying still on the ground allows the next two bats to fly over the Knight without being damaged.
 * Dive Dash: Dash, walk, or jump to avoid the Dive once it starts. It is best to jump over the Dash. The Knight can also time a down strike on Nightmare King while he is dashing, or Great Slash to stall in the air and hit Nightmare King Grimm as he stands up. Double jumping, while Nightmare King Grimm is dashing, allows the Knight to stay aloft long enough to avoid the flame trail. Be careful when Nightmare King Grimm finishes his dash and stands up, as his hitbox grows a bit higher and may cause contact damage to the Knight even if they seemingly dodged the flame trail.
 * Dash Uppercut: If possible, dash away from Nightmare King Grimm, then attack or Great Slash him when he finishes his dash and before his leap into the air (or hit him with Abyss Shriek). If there is not enough room in the arena to dash away from him, the Knight needs to pause and wait for about half a second before trying to shadow dash through him. Otherwise, he skitters away and leaves the Knight in a precarious position (to avoid damage in that scenario, jump straight up as quickly as possible). If Sharp Shadow is equipped, due to the increased dash distance, it would be wiser to dash straight through Nightmare King Grimm. After dashing through him, dash back to the spot the Knight came from, which is now one of the empty spots, in between the incoming fireballs of the second part of Nightmare King Grimm's attack.
 * Flame Pillar: There are many options here. Slowly move in one direction so pillars erupt behind the Knight, try avoiding being stuck in a corner of the arena (if necessary Shadow Dash through a Pillar). With this method, try to strike Nightmare King Grimm a couple of times with the Nail, and/or use Great Slash. It is possible to double jump and Nail-bounce on Nightmare King Grimm, moving left or right as necessary to avoid the next Pillar(s). While hovering still, Nightmare King Grimm leaves himself open for a full-damage Abyss Shriek. Be careful that hitting Nightmare King Grimm from sides causes knockback and may accidentally push the Knight back to the burning pillar behind, and hitting from below may cause over-jump and deal contact damage.
 * Pufferfish: It is best to stay close to the wall but not to stick to it as the Knight 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 Knight does not accidentally hit the fireballs flying over their head. Another way is to climb the wall and jump onto to Nightmare King Grimm and Nail-bounce on him. Though to do this, the Knight must take damage to take advantage of the invincibility frames or be really quick as soon as he starts the attack. With proper timing and a bit of luck, Shade Soul and Abyss Shriek can be useful here.

Healing : Without any healing charms, the Knight can heal at some rare moments:
 * Once right after the last flame pillar begins to form.
 * Once by jumping over one firebat and then immediately healing (the flight pattern for the next low firebat is altered by its homing ability, causing it to fly over the Knight's head, allowing time to heal possibly even twice) or as quickly as the Cloak Spikes attack starts though the timing on this combined with the positioning aspect makes it more tricky.
 * It is possible to heal once in the Cloak Spikes attack if the Knight is lucky enough to stand on safe ground right from the get-go and immediately heal before the spikes start to thrust up.
 * It is possible to heal once if the Knight dashes away (not through, as it is harder to predict the falling fireballs and waste the healing window) from Nightmare King Grimm when he is performing the Dash Uppercut attack. Nightmare King Grimm cuts right behind the Knight and the centre fireball misses as well, creating a healing window.
 * Three heals are also possible during the stagger as long as the Knight 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 Knight immediately starts healing once the attack starts (as the animation to transition into a slug takes enough time that healing too late causes the Knight to be hit instead). In addition, healing during the Cape Spikes attack 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 combination of Quick Focus and Deep Focus allows the Knight to heal 2 Masks at a time during all of these no-charm healing opportunities.

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 is easier to hit Nightmare King Grimm from farther distances. Sharp Shadow is a double-edged sword if used with improper spacing because the Knight may end their dash in Nightmare King Grimm, resulting in taking damage.
 * Fragile Strength / Unbreakable Strength, and Mark of Pride can be very helpful for reaching and damaging him. As Nightmare King Grimm is a very elusive boss and leaves a very small window to hit, it is recommended to use Nail Arts to maximize damage output, so Nailmaster's Glory is a cheap and useful choice.
 * The Minion Build is a popular option, allowing Knights to focus on dodging. Weaverlings can damage Nightmare King Grimm during the Pufferfish attack, as well. The Minion build (in full) is Weaversong, Glowing Womb, Grubsong, Sprintmaster, and Defender's Crest (as well as Grimmchild).

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

Trivia

 * Other than the Radiance and Absolute Radiance, Nightmare King Grimm is the only other boss to have a full-screen title card at the beginning of its fight.
 * When fighting Nightmare King Grimm in Godhome, his title card appears on the bottom of the screen, making him the only boss to have both a regular title card and a fullscreen title card.
 * The Grimmkin Nightmares watching in the background rear back and 'laugh' whenever Nightmare King does the Pufferfish attack and after Nightmare King Grimm has been defeated.
 * 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.
 * Nightmare King also has 6 legs when he uses the Skitter manoeuvre compared to Grimm's 4.
 * An alternative way to predict Nightmare King Grimm's health is to observe the background, where the Nightmare's Heart sits, as it changes per each Flame Spawner attack. At 75% health, the Heart beats faster, at 50% the Heart's eyes glow red and at 25% the Heart beats ever so faster and the glow becomes more intense.
 * A red barrier appears over Grimm when trying to enter the fight without Grimmchild equipped.
 * In Godhome, it is not required to have the Grimmchild charm equipped to fight Nightmare King Grimm, which is also true for Troupe Master Grimm. If it is equipped, the Grimmchild is not present for the fight.
 * While Nightmare King Grimm can be fought in the Pantheon of Hallownest or the  Hall of Gods if the Grimm Troupe has been banished, his Hunter's Journal entry is not unlocked after defeating him there. This means that his entry can never be obtained after banishment.

Король кошмара Grimm Re dell'Incubo Król Koszmarów Grimm