A tough, furred seed hidden deep in the Sands of Karak, and one of five foods the Great Gourmand wants.
The Crustnut is the delicacy that sends players in circles, because it is buried behind a wall of breakable terrain in one of Silksong's least obvious rooms.
It is a Deliverable item, not gear. The wiki describes it as a "tough, furred seed from the centre of a corpora cluster," stale to taste but strong once ground as seasoning.
Its whole purpose is the kitchen. The Crustnut is one of the five ingredients the Great Gourmand wants, which means it ties directly into a single wish rather than any combat upgrade.
This guide covers what the Crustnut is, exactly where it hides in the Sands of Karak, who wants it, and the mistakes that keep players from finding it.
A Deliverable seed, a Pharloom delicacy.
Sands of Karak, behind breakable walls and floors.
Loyal Mergwin and the Great Gourmand.
Great Taste of Pharloom wish ingredient.The Crustnut is a Deliverable, a quest item Hornet carries to hand over rather than use in a fight. According to the wiki, it is a tough, furred seed pulled from the centre of a corpora cluster.
The flavour text leans into the joke of it. The wiki notes it is stale to taste on its own but strong once ground down as seasoning, which is exactly why a master cook would want it.
It carries no combat value. Unlike a 
tool or upgrade, the Crustnut exists purely to satisfy a single gather quest.
The Crustnut sits in a hidden room in the Sands of Karak, behind many breakable walls and floors. Per the wiki, you have to smash through a stretch of false terrain to reach the enclave it is tucked inside.
The Sands of Karak is the same Act 3 region that holds the Coral Tower and the boss Crust King Khann, so you will likely be in the area for that fight anyway. Fold the Crustnut hunt into that trip.
Loyal Mergwin hints at the location himself. The wiki quotes him recalling that "scores of them could be gathered, high atop those coral crusted caves beyond the Grand Gate," which points you toward Karak's coral rooms.
The Crustnut's only job is to feed the Great Gourmand. It is one of five delicacies Loyal Mergwin asks Hornet to gather on his master's behalf, and it has no other use in the game.
You deliver it to Loyal Mergwin in the Choral Chambers. He is the broker for the whole feast, collecting each food as Hornet brings it back from across Pharloom.
Because it is a single-use Deliverable, there is no reason to hold onto it. Once you have it, the next step is the handover.
The Crustnut is one fifth of the Great Taste of Pharloom wish, the gather quest that rewards Pale Oil and the Glutton achievement.
The other four foods are the Mossberry Stew, Vintage Nectar, Courier's Rasher, and the Pickled Muckmaggot from the gauntlet on 
Sinner's Road. The Crustnut is usually the trickiest of the five to track down.
In actual play, the seed is the one most players get stuck on, simply because nothing on the surface of Karak signals the breakable room. Knowing it is there is most of the battle.
The Crustnut is one stop on a wider feast. These guides cover the wish, the region, and the cast around it.
Game data and screenshots adapted from hollowknight.fandom.com, used under CC BY-SA 3.0. Original content remains the property of the wiki contributors and Team Cherry.