Once you reach the late game section called The Slab in Hollow Knight: Silksong, you might encounter several locked areas. To access them, hunt around for the right keys and use them to gain access to the next areas. The Key of Heretic is one of the core keys to locate, along with the Key of Indolent. This guide will explain how to get the Key of Heretic and where to use it in Hollow Knight Silksong.
Key of Heretic Location Hollow Knight Silksong

The Key of Heretic is found after defeating certain enemies at The Slab. First, you need to reach the very top of the Slab, then take the exit on the left to drop down to the lower sections. Our detailed map above will show you where you need to go and how to navigate the Slab.
Once you reach the lower level, you will spot a section of the platform where you get the Descend notification. Get down that area and fight the bugs here to get the Key of Heretic off their corpses.
Where to Use the Key of Heretic in Hollow Knight Silksong?

The Key of Heretic is one of the three Slab Keys used to unlock the various cells and passages within the Slab. To find all Heretic doors, head east after defeating the enemies and rest on the bench. Inspect the door beyond the bench to unlock it using the Key of Heretic. Keep going east, and you will come across another locked door that this time requires the Apostate key.
We are, however, not done with using the Heretic Key yet. Ignore the above locked door, and keep descending down, you will eventually come across another door that has ‘A lock with the mark of a Heretic’. This is your cue to use the Heretic Key once more. This opens the floor below. Keep going down till you reach a dead end. Go west from there and drop down the narrow passage. Continue down and go east, break down the wall, then continue to go east and keep breaking walls. You will reach a dead end with the Apostate door, drop down the ditch beside it, and go west.
Drop down the final ditch and continue down till you reach the bottom, then go west to finally unlock a fast travel point for The Slab area in Hollow Knight Silksong.