Nosk

The Nosk is a secret boss found in Deepnest that guards a Pale Ore in the room beside it.

Location
A breakable wall in the room directly left of the Deepnest Hot Springs will reveal a secret section that contains a Grub. Breaking another wall behind the Grub will take you to the tunnels that lead into the Nosk's lair.

Physical Description
The Nosk when first encountered looks exactly like the Hollow Knight.

However, when it reveals its true form, it resembles a huge spider-like creature with spikes on its back and a orange pus-filled belly. It also wears the disguise it was using to trick the Hollow Knight on it's head, with the head flipped to make the horns resemble scythe-like jaws.

Attacks
The Nosk can charge at you, screeching to warn the player before it charges.

The Nosk can also jump around in a sporadic way.

Tactics
While it may not seem difficult with only two attacks, the Nosk is speedy, leaving little time to heal. Quick Focus is recommended as always. Try shadow dashing if you have access to it, although it's very possible to do this fight before then.

There is a small elevated platform at the center of the Nosk's battlefield. Standing just beside it will significantly reduce the risk of the Nosk hitting you, providing plenty of time to heal.

Trivia

 * Using the Dream Nail on the many victims found throughout the corridors will reveal that the Nosk took the shape of their lost loved ones to trick them.
 * The Nosk's room has many webbed-up victims hanging from the ceiling, including a massive Stalker Devout. Curiously, some of its victims appear to be Vessels, like the main protagonist.
 * When it dies, the Nosk drops the same mask the Hollow Knight drops upon death.
 * The Nosk may be a reference to The Thing (from the famous movie of the same name), as it shape shifts to lure its victims closer, and does so in a very gruesome fashion. It also could be inspired by the SA-X from Metroid.
 * Unusually for a bonus boss that can be easily missed, the Nosk has its own theme music.
 * The first time you fight the Nosk, you will see a disturbing intro sequence of it transforming from its Hollow Knight form into its true grotesque appearance.
 * It's possible the Nosk is the boss of Deepnest, as you don't fight any other boss there.