Flukemarm

Flukemarm is a hidden boss in Hollow Knight. Defeating her grants a new Charm.

Lore
Flukemarm is the mother of all Flukes infesting the Royal Waterways. Her body is used as a nest for young Flukefeys. Like her children, she was infected and made hostile, although her condition as breeder prevents her from leaving her cave.

Hollow Knight
Flukemarm can be found suspended in a hidden cave below the pipework of the Royal Waterways. She lays dormant at first, but will show hostility as soon as the Knight strikes her. Her defense relies on throwing Flukefeys at her foe.

When Flukemarm is defeated, the Infection within bursts out, making her explode. She leaves behind behind the Flukenest charm.

Behaviour and Tactics
Flukemarm has the following singular attack: An optional boss that is located in the centre of a massive room. She will remain passive and idle until provoked by damage. The Knight will also not be locked into this fight, meaning the player is free to escape anytime during.
 * Spawn: Flukemarm will spit out two Flukefeys from either of its orifices telegraphed by bile spewing out of the selected ones. There can only be a maximum of 6 Flukefeys spawned at once.

An "offensive" tactic requires upgrading the Nail enough (Nail Upgrade 3 or Higher) to kill Flukefeys in one hit or using Fragile Strength / Unbreakable Strength. Focus on killing the Flukefeys as quickly as possible and hit the Flukemarm between spawns.

Another "offensive" tactic is to gain SOUL from the Flukefeys, and then using Vengeful Spirit or Shade Soul to deal damage to Flukemarm continuously.

A "defensive" tactic involves using Quick Focus and Stalwart Shell to buy time, Soul Catcher and Grubsong to generate enough SOUL to heal, and Thorns of Agony to help kill the Flukefeys and occasionally damage the Flukemarm itself if the player is close enough to it.

Using Desolate Dive/Descending Dark is effective as the Spell gives enough invincibility to repeatedly use the spell without taking damage.

The most important thing to avoid in this fight is falling into the water. Since the player cannot attack while swimming, they will be left completely defenceless.

Trivia

 * Flukemarm used to only use the ambient theme for the Royal Waterways. It has since been changed to the action theme.