The Key of Apostate is required to open late-game gates in The Slab. Despite the label, it isn’t found inside The Slab at all—the key sits in Putrified Ducts. Below are two reliable routes to reach it, the exact spot to pick it up, and where to use it once you have it in Hollow Knight: Silksong (official site).

Requirements and helpful abilities

  • Faydown Cloak (double jump) is required to reach the entry points and traversal gaps.
  • Cling Grip (wall jump) and Clawline (grapple) make the platforming consistent, especially on the Bilewater side.
  • Have a nearby bench/bellway marked before you set out to limit runbacks through Putrified Ducts.

Route 1: From Memorium (Choral Chambers/Citadel side)

Step 1: Reach Memorium via the upper path from Choral Chambers/Songclave once you have double jump. Look for a ceiling opening that lets you climb into the area.

Step 2: Traverse to the far right of Memorium until you step back outdoors. Cross the long upper bridge while fending off flying enemies.

Step 3: At the bridge’s far end, jump up to find a hidden opening and breakable rock. Break through to open the entrance into Putrified Ducts.

Step 4: Inside Putrified Ducts, locate the main vertical shaft in the adjacent room. Drop to the bottom of the shaft to reach the lower passages.

Step 5: Head left at the base and climb into the dim side tunnel that continues left. At the tunnel’s far end, smash the cage to obtain the Key of Apostate.

Route 2: From Bilewater (after clearing the upper boss)

Step 1: From Bilewater’s upper section, defeat the boss blocking the high passage. Cut the rope/activate the shortcut above the arena to link into the Ducts side.

Step 2: Move left into the spike-lined vertical corridor and climb carefully using double jump and wall clings. Use Clawline tethers to cross wider gaps or avoid ranged pests.

Step 3: Enter the Putrified Ducts and orient on the main vertical shaft in the next rooms. Keep to the western side paths to reach the lower tunnels.

Step 4: Take the leftward tunnel and watch for ambushes in the dark. At the far left, strike the glowing cage to claim the Key of Apostate.

Tip: The Ducts’ narrow halls include enemies that grab as you approach; poke from reach or bait their tongues before committing to jumps.

Where to use the Key of Apostate in The Slab

What it unlocks: Two Apostate locks inside The Slab—one that opens the main route if you weren’t kidnapped, and a second that gates a platforming room with a reward.

Step 1: From The Slab’s bellway and nearby bench, climb into the right-hand wing to find the Apostate-locked door. Use the key to open a direct entry into the complex without needing a jailer capture.

Step 2: After entering, climb to open the convenient shortcut above before dropping back down to explore. This reduces future backtracking across the entry gauntlet.

Step 3: Continue upward from the Heretic-lock corridor toward the bench signage. At the top, use the key to enter a platforming puzzle room.

Step 4: Press the floor button four times to spawn four moving wheels. Double jump to the first ledge and time your jumps to land on wheels as they pass.

Step 5: Use wheel momentum to cling to the opposite wall, then chain two more wheel jumps to reach the upper platform. Collect the mask shard at the top.

Optional content: The Apostate door above the Slab’s bellway also leads deeper into darker tunnels. Progress there opens an optional route that culminates in a hidden boss encounter.

Tips and troubleshooting

Not captured by the jailer? You are not locked out. The Apostate door near the Slab’s bellway/bench serves as the legitimate way in for players who never triggered the kidnapping sequence.

Having trouble finding the Memorium break? In the eastern rooms, look up for a cracked ceiling/hidden opening and listen for the subtle break sound when striking suspicious tiles. The climb feeds directly into the Ducts entry.

Surviving Bilewater. The water and swarms can chip away quickly. Clear rooms methodically, and consider crests that mitigate damage-over-time or improve recovery to stabilize long ascents.

Minimize runbacks. As soon as you reach a new region’s bellway or a safe bench, activate it. Putrified Ducts connects back to multiple hubs, so anchoring a checkpoint saves many minutes if you miss a jump or get grabbed.

Once the key is in hand, The Slab opens up quickly: the bellway door streamlines entry and the upper lock nets a permanent health upgrade. Bring double jump and patience—both routes to the Ducts are tough but consistent once learned.