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The Key of Apostate is the last key you won’t find while escaping the Slab in Hollow Knight: Silksong. This last key hides a huge secret — one of the toughest bosses in the game. The optional First Sinner is an impressive super boss that’s also totally optional and very easy-to-miss if you aren’t exploring every corner of the map. If you want to fight everything Silksong has to offer and want an even more difficult experience, here’s how to find the First Sinner.
Before you can access the First Sinner, you’ll need to unlock and escape from the Slab. The prison secret level is very creative, and it’s the only way to get the Heretic Key you’ll need to access the First Sinner’s chamber.
Where To Find The Key of Apostate
The Key of Apostate is located in the Putrefied Ducts secret level. Learn how to reach the Putrefied Ducts with our full guide here.
- Key of Apostate Location: Go to the south of the Putrefied Ducts to reach an area that’s directly above the Bilewater region. Go west to find an extremely dark long tunnel — a large green slime beast will ambush Hornet in this tunnel.
- Get past the monster, and a tongue monster will grab Hornet. Let the monster grab you and take you up to a higher level — you can’t reach the area above with wall-jumps or double-jump. You have to let the tongue monster grab you.
- Break free after it pulls you up, then go left (west) to the end of the passage. You’ll find glowing key item here and a dead barnacle tongue monster. That’s where you’ll find the key.
The Key of Apostate is used at the Slab, a unique secret area that can only be accessed if Hornet is captured by a monster carrying a cage in the areas outside the Citadel.
After being caught, Hornet will spawn at the Slab. You’ll need to escape and reclaim your gear before you can continue the game. The only key you won’t find while exploring the Slab is the Key of Apostate.
Where To Use The Key of Apostate
The Key of Apostate can be used in two locations in the Slab.
- Location #1: Use the Key of Apostate on the door near the Bellway. Go up from the Bellway and right to find a locked gate.
- You’ll find a room with a record of executions. There’s a hidden path above — inside the cave, there’s another breakable wall on the right side of the tall shaft that contains a Memory Locket. This path also creates a shortcut through the Slab.
This area also unlocks a gate in the lower level of the Slab. This underground area has a Key of Heretic door. The Key of Heretic is found when escaping the Slab originally.
- First Sinner Boss Room: The locked door leads to a strange chamber with a cell covered in chains. Break all the chains to enter — interact with the character inside to initiate a boss fight against the First Sinner.
This is a very aggressive optional super boss that may be overwhelming at this stage of the game. The First Sinner uses an array of attacks and moves faster than any other boss you’ll have encountered at this stage of the game.
- Location #2: The door is located directly above where Hornet first appears after being captured and taken to the Slab. It’s on the top-right of the central vertical shaft.
- The door leads to a challenging platforming room where Hornet presses a button to summon spike balls on rails. You’ll need to bounce off the spike balls by jumping off them with attacks, following them up to the top.
The second (upper) Apostate Key door leads to a difficult platforming challenge. You’ll find a Mask Piece reward at the top of this area.
