What is the Key of Apostate?
So you've been exploring The Slab and found two locked doors that won't budge? That's because you need the Key of Apostate. Here's the catch: unlike the other two keys you can find in The Slab, this one's hidden way across the map in Putrified Ducts. Yeah, that horrible place with the grabby vines. Behind these doors? A tough boss fight that rewards you with Rune Rage, and a tricky platforming challenge with a Mask Shard at the end.
Quick Reference
Location:
Putrified Ducts (upper-left section)
Required Abilities:
Faydown Cloak, Cling Grip
Usage Location:
The Slab (2 doors)
Difficulty:
Medium - Dangerous area
Prerequisites Checklist
Don't even think about going after this key unless you have:
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Faydown Cloak:
You get this double jump from beating Stairways at Mount Fay. Without it, you literally can't reach the key.
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Cling Grip:
The wall jump ability. You'll need this for climbing through all those spike-covered vertical sections.
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Swift Step (Recommended):
Technically optional, but trust me—the dash makes this SO much easier when dealing with enemies in tight spaces.
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Wreath of Purity (Recommended):
Going through Bilewater? You'll want this to deal with the contaminated water and those annoying Muckmaggots.
Where to Find the Key of Apostate
Location Overview
Overworld Location
Detailed Map Location
The key is in Putrified Ducts—you know, that gross area in the top-right corner of the map. Specifically, it's in the upper-left corner of a big room in the lower section. Yeah, it's basically on the opposite side of the map from where you need to use it. Team Cherry really wants you to work for this one.
Heads up: Putrified Ducts sucks. Like, really sucks. Those carnivorous vines will grab you if you're not careful, enemies are everywhere, and the whole place is just nasty. Bring extra healing and maybe upgrade your health first if you're struggling.
Route 1: Access from Bilewater (Recommended)
This is the faster way, but you'll have to deal with Bilewater's toxic water and some really tight vertical climbing sections. Here's how to get there:
How to Get There:
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Step 1: Start at Bilewater Bench
Begin at the bench checkpoint located west of Groal the Great in Bilewater. Equip the Wreath of Purity if you have it to protect against water hazards.
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Step 2: Navigate to the Vertical Shaft
From the bench, proceed left (west) until you reach a vertical pathway with spikes on both walls. This is your entry point to climb upward toward Putrified Ducts.
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Step 3: Climb Using Faydown Cloak and Cling Grip
Use your Cling Grip to latch onto the walls and double jump with Faydown Cloak to navigate between the spike-covered surfaces. Time your movements carefully to avoid taking damage.
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Step 4: Find the Tunnel Entrance
Once at the top of the vertical climb, look for a tunnel-like passage on the left side. This leads into the upper section of Putrified Ducts.
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Step 5: Clear Enemies and Continue Left
Enter the tunnel and defeat any enemies you encounter (primarily Barnaks and Ductsuckers). Keep moving left through the narrow passages.
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Step 6: Cross the Gap
You'll reach a gap that requires precise movement. Use Sprint combined with your dash (Swift Step) to safely reach the other side. If you don't have Swift Step, a well-timed double jump may suffice, but it's much more difficult.
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Step 7: Retrieve the Key
At the end of this path, you'll find a broken cage containing a glowing object. Strike the glowing light repeatedly to break it free and obtain the Key of Apostate.
Pro Tip: The enemies here get super annoying in tight spaces. Your Bind attack (hold the attack button) is great for crowd control since it hits multiple enemies at once. Way safer than trying to dodge everything individually.
Route 2: Access from Memorium
An alternative route exists through the Memorium region, accessed from the Choral Chambers. This path is longer but may be preferable if you haven't explored Bilewater thoroughly yet.
Step-by-Step Walkthrough:
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Step 1: Reach the Memorium
From Choral Chambers, take the upper exit to reach the Memorium area. You'll need the Faydown Cloak to access the higher platforms.
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Step 2: Find the Hidden Wall
Navigate to the right side of Memorium until you reach a long bridge that appears to lead nowhere. Look for a breakable wall on the right side.
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Step 3: Break Through to Putrified Ducts
Use Cling Grip on the wall and jump upward with Faydown Cloak to reach the upper section. Attack the breakable wall to create an opening into Putrified Ducts.
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Step 4: Enter and Navigate Right
Once inside Putrified Ducts, head right while avoiding or defeating enemies. Watch out for the carnivorous vines hanging from the ceiling.
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Step 5: Descend to the Large Chamber
After moving through the horizontal section, you'll reach a vertical area. Drop straight down to the underground section.
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Step 6: Find the Left Tunnel
In the underground chamber, look for a tunnel passage leading to the left. This is also where you'll meet the Huntress NPC for the Broodfeast quest.
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Step 7: Navigate Past Tongue Enemies
Continue left through the tunnel. You'll encounter enemies that grab you with their tongues—dodge these carefully or eliminate them from a distance.
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Step 8: Claim the Key
At the left end of the chamber, you'll find the Key of Apostate enclosed in a cage. Break it to obtain the key.
Don't Make These Mistakes
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Going Too Early: I tried this without Faydown Cloak. Don't be like me. You literally cannot make some of these jumps without double jump. Save yourself the frustration.
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Running Past Enemies: Yeah, the spaces are tight and you want to just zoom through. But getting cornered by 3 enemies at once in a narrow tunnel? That's how you die. Take it slow.
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Forgetting to Heal Up: There's no bench in Putrified Ducts. Like, at all. Make sure you're topped off before heading in.
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No Map Marker: Drop a pin when you find the key location. Trust me, you'll want to come back for other stuff later and you will NOT remember where it was.
Where to Use the Key of Apostate
Got the key? Great! Now haul yourself back to The Slab. There are two doors with purple locks waiting for you. Each one has something good behind it, so let's go over what you're getting into.
Door #1: First Sinner Boss Chamber (Lower Slab)
Door Location in The Slab
Map Location
Location Details:
This door is located above the room where you'll find the Bellway (fast travel point) and the area's bench in The Slab. From the bench room, you'll need to climb upward to reach this door. It provides access to the lower prison areas of The Slab.
What's Behind the Door:
This door leads to a creepy prison cell full of chains. Break the chains and boom—boss fight time. The First Sinner is no joke. She's one of the tougher bosses in The Slab.
Boss Fight: First Sinner
- Difficulty: Hard - She's fast and hits like a truck
- Attack Patterns: Teleporting strikes, AoE magic attacks, and those annoying chain whips
- Strategy: Play it safe. Dodge first, attack second. Getting greedy = getting dead.
- Reward: Rune Rage - Super powerful Silk ability that buffs your damage
Tip: Don't try this boss until you have at least 6 masks (full health bar) and some decent Weaver Seals equipped. Practice your parry timing too. This fight is all about patience—resist the urge to go ham on her when she's vulnerable.
Door #2: Mask Shard Platforming Challenge (Upper Slab)
Upper Door Location
Map Position
Location Details:
This door is found in the upper-right section of The Slab's main shaft, significantly higher than the first door. You'll need to climb or navigate through several platforms to reach it.
What's Behind the Door:
This one's all about platforming. You've got a vertical gauntlet with rotating spiked wheels that you need to pogo on to climb up. It's tricky, but the Mask Shard at the top is worth it.
Platforming Challenge Details
To complete this challenge and reach the Mask Shard at the top, you must:
- 1. Activate the Mechanism: Strike the lever/switch to set the spiked wheels in motion.
- 2. Time Your Pogo Jumps: Downward strike (pogo) on the rotating wheels while they're in safe positions.
- 3. Chain Multiple Pogos: You'll need to bounce between several wheels in sequence without touching the ground or spikes.
- 4. Reach the Top Platform: The Mask Shard awaits on the highest platform after successfully navigating all wheels.
Reward:
Mask Shard
Collecting this Mask Shard contributes toward increasing Hornet's maximum health. Every 3 Mask Shards collected grants one additional health mask, making this a valuable reward for completing the challenge.
Pro Tip: If you suck at pogo-jumping (like I did), go practice on some regular enemies first. The timing windows here are TIGHT. Don't feel bad about coming back later with more upgrades if you're struggling—this challenge isn't going anywhere.
Advanced Tips and Strategies
Combat Tips
- Ductsucker Strategy: These enemies grab Hornet with their tongues. Keep moving and attack from diagonal angles to avoid their grab.
- Barnak Handling: These ceiling enemies drop on you. Listen for audio cues and watch for visual tells before they attack.
- Carnivorous Vines: The plant tentacles in Putrified Ducts can be stunned temporarily. Hit them once to create a safe window for passage.
Navigation Tips
- Map Markers: Mark the key location and both door locations on your map for future reference.
- Backtracking Route: After getting the key, the fastest return to The Slab is usually the way you came, not continuing through Putrified Ducts.
- Bench Planning: The nearest bench to the key is in Bilewater. Plan your health management accordingly.
Optimal Progression Order
- 1. Explore The Slab fully and obtain the Key of Indolent and Key of Heretic first
- 2. Complete Mount Fay to obtain the Faydown Cloak double jump ability
- 3. Upgrade your health to at least 6 masks before attempting Putrified Ducts
- 4. Retrieve the Key of Apostate using the route that suits your current exploration progress
- 5. Return to The Slab and decide whether to tackle the boss fight or platforming challenge first based on your play style
- 6. Consider attempting the Mask Shard challenge before the First Sinner if you want the extra health for the boss fight
Troubleshooting Common Issues
"I can't reach the vertical shaft in Bilewater"
Solution: Ensure you have both Faydown Cloak (double jump) and Cling Grip. Without both abilities, the spike-covered climb is impossible. Also verify you're at the correct vertical shaft—it should have spikes on both walls.
"I keep dying to the enemies in tight spaces"
Solution: Use your Bind attack (hold attack button) to create space and deal damage simultaneously. Equip Weaver Seals that provide defensive benefits or area damage. Don't be afraid to retreat and heal—there's no time limit.
"I can't find the breakable wall in Memorium"
Solution: The wall is on the right side of the long bridge area. It looks slightly different from regular walls—look for a subtle crack pattern. Try attacking all suspicious walls in the area.
"The pogo challenge for the Mask Shard is too difficult"
Solution: Practice makes perfect. The timing is strict but learnable. Watch the wheel rotation patterns carefully—there's a rhythm to it. You can also return later after obtaining additional movement abilities that make the timing more forgiving.
"I already used the key but can't remember where"
Solution: The key opens both doors simultaneously—you don't need to choose. Once obtained, simply approach either locked door in The Slab and it will unlock automatically. Check both door locations shown in this guide.
Related Guides and Content
All Mask Shard Locations
Complete guide to finding all Mask Shards in Silksong, including maps and difficulty ratings for each location.
The Slab Complete Walkthrough
Full exploration guide for The Slab region, covering all three keys, boss fights, and hidden collectibles.
Faydown Cloak Guide
Learn how to obtain the Faydown Cloak double jump ability, a prerequisite for reaching the Key of Apostate.
Bilewater Region Guide
Explore the contaminated waters of Bilewater, one of the access routes to the Key of Apostate location.
Wrapping Up
The Key of Apostate is a pain to get—no sugarcoating that. Putrified Ducts is gross, the enemies are annoying, and it's on the opposite side of the map from where you need it. But hey, the rewards are solid: Rune Rage is one of the best Silk abilities in the game, and you always need more Mask Shards.
Take your pick on which door to tackle first. Want the boss fight? Go for the First Sinner. Prefer platforming? Hit up the Mask Shard room. Either way, make sure you're actually ready before diving in. There's no shame in coming back later when you're stronger.
Now go unlock those doors and claim your rewards!
Guide last updated September 2025