The Apostate Key in Hollow Knight Silksong is the final key needed to access The Slab, and locating it requires navigating a tough area.

Where to Locate the Apostate Key in Silksong

After you escape The Slab, you may notice several doors still locked in the prison. If you’ve thoroughly explored the surrounding areas and are unable to find the last key, it's important to know that the Apostate Key is situated in a different location.

Silksong features a highly non-linear Metroidvania design. As you advance through Pharloom, you will revisit various locations on the map. To acquire the Apostate Key, you must access the Putrified Ducts, which only becomes available later in your journey.

Apostate Key Location

The Apostate Key is located within the Putrified Ducts, which can be entered from either Bilewater or Memorium, depending on your progress in the game. Regardless of the path you take, you will need to have unlocked the double jump ability (Faydown Cloak) to reach the required platforms.

Bilewater Route

Start at the bench in the main boss arena of Bilewater. Move left into the adjacent area, then ascend the vertical section filled with spikes. Enter the Putrified Ducts and continue your ascent. Take a left turn and attack the tentacles hanging from the ceiling. Once you arrive at a broken cage with a glowing light inside, strike it until you obtain the Apostate Key.

Memorium Route

Begin your journey in The Citadel and navigate through Memorium. This route is longer but bypasses Bilewater. Look for a concealed passage above the rightmost wall in one of the connecting areas, identifiable by a flower marked on the map. With the Apostate Key acquired, you can return to The Slab. If you head west from the Citadel Spa, you will even encounter the map for The Slab along the way.

Additional Unlockables

If you are on the lookout for more secrets, be sure to consult the guides for Silksong Simple Key and door locations. If you find yourself struggling with difficulty, gather Mask Shards and enhance your needle to increase your chances of success.