Flukemarm

Flukemarm is an optional boss in Hollow Knight. It is the parent of the multitude of flukes that flourish in the Royal Waterways of Hollownest. Following the Infection, Flukemarm gained the obsessive goal to breed and repopulate the dying kingdom with its young, even if it means killing off the other species to dominate.

Appearance
Flukemarm is a large silver-colored fluke worm covered in pink orifices and small tentacles. Because it is unable of movement, Flukemarm attaches itself to the ceiling with a clear substance.

Battle
Flukemarm is located in the bottom-left corner of the Royal Waterways, hidden behind a secret breakable wall in the room with flukes and Flukemons. At first, Flukemarm is resting and will not attack the player until it is attacked first.

Flukemarm itself does not attack the player, but instead summons two aggressive Flukefeys from its orifices at a time following a span of a few seconds. Flukemarm signals which orifices will spit out Flukefeys by spewing out infection before spiting them out. While the Flukefeys can be defeated in one or two hits, their rapid movements will quickly outnumber the player if they focus on fighting Flukemarm or concentrate on healing, so constant movement is recommended to avoid damage. One strategy is to have the Nail upgraded high enough to kill the Flukefeys in one hit, and then hit Flukemarm in-between waves.

The most important thing to avoid in the fight is the water. Since the Knight cannot attack while swimming, they are completely vulnerable to attacks.

Defeating Flukemarm drops the Flukenest charm, which when equipped changes the Vengeful Spirit and Shade Soul spells into a horde of volatile baby flukes.

Trivia

 * Flukemarm is the only boss who lacks movement.
 * As of the Lifeblood update, using the Desolate Dive spell on Flukemarm no longer functions, as the Knight will simply bounce off Flukemarm's body when used.