You’re stuck in The Slab. Honestly, it’s one of the most frustrating moments in Hollow Knight: Silksong if you weren't ready for it. One minute you’re platforming through Greymoor, and the next, some bulky, cage-carrying fly nabs you and drags you to a freezing prison. You’ve lost your needle. You’ve lost your cloak. You’re punching enemies because you have no other choice.
Naturally, you start scouring the cells. You find the Key of Indolent. You find the Key of Heretic. But then you see that third lock. The one for Silksong the slab apostate key.
Here is the cold, hard truth: you aren't finding that key in the prison. Stop looking. You’re wasting your time hitting walls that won't break and jumping into spikes that just send you back to Grindle’s bench. The Apostate Key is one of Team Cherry’s most devious "come back later" items. It is literally on the other side of Pharloom.
The Wild Goose Chase for the Apostate Key
Most players assume that because the lock is in the jail, the key must be in the warden's pocket. It makes sense, right? Wrong. The Silksong the slab apostate key is actually located in the Putrified Ducts.
If you're currently trapped in the prison section of Act 1, your priority isn't the Apostate Key. It's getting your gear back and finding the Bell Beast station to leave. You’ll see the Apostate locks near the vertical corridors and the Choral Chambers shortcut, but you have to ignore them for now. It’s a psychological trap. You feel like you're missing 100% completion in the area, but the game is actually forcing you to progress into the Citadel and beyond before you can even dream of touching that key.
To even reach the location where the key is hidden, you need two major upgrades:
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- The Faydown Cloak (your double jump, found at the top of Mount Fay).
- The Clawline (the harpoon-style grapple).
Without these, the Putrified Ducts will just be a series of ledges you can't quite reach.
How to Actually Find the Key
Once you’ve finally secured your upgrades and made it into the Citadel during Act 2, you can start the hunt. The easiest way into the Putrified Ducts is through the Memorium area. It's tucked away north of the Choral Chambers.
Look for a room filled with tattered rags hanging from the ceiling. It looks like a dead end, but if you have the double jump, there’s a hidden opening in the roof. From there, you’ll cross a bridge where a swarm of bats—the ones that always seem to hit you right as you're mid-dash—will try to knock you into the pit.
The path is a bit of a labyrinth. You’ll go through the Ducts, avoid the "Ductsucker" enemies (which are exactly as gross as they sound), and eventually find a breakable wall on the far left of the map.
Expert Tip: Don't just rush in. There’s a specific spot where a tentacled monster and a wardenbug are tangled together in a pile of junk. It’s pretty grim. You have to attack the glowing item stuck in that pile to finally get the Silksong the slab apostate key.
Why Bother Going Back?
You might ask if it's even worth the trek. You’ve already escaped the prison. Why go back to a place that took your clothes and your weapons?
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The rewards are actually some of the best in the mid-game.
The first Apostate lock you’ll likely use is the one near the Bellway. It opens a massive shortcut to the Choral Chambers. If you’re trying to navigate back and forth for the various "Wishes" (Silksong's version of side quests), this shortcut saves you about ten minutes of backtracking through the Citadel's annoying industrial sections.
But the second lock is the real prize. It's located at the very top of a vertical corridor in The Slab. Inside, you’ll find a platforming challenge that leads to a Mask Shard. More importantly, there’s a dark tunnel tucked behind a hidden wall in that same room.
The First Sinner Boss Fight
This is the part most people miss. Using the Silksong the slab apostate key eventually leads you to an optional boss called The First Sinner.
It’s a brutal fight. The boss uses a heavy chain-and-sickle style attack that covers half the screen. It’s also one of the few places where you get significant lore about why the prison exists in the first place. Apparently, the "Apostates" weren't just common criminals; they were bugs who refused to participate in the "shining" of the Citadel.
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If you beat The First Sinner, you don't just get the satisfaction of winning. You get a Weaver Effigy that you can hand in to Scrounge in Bellhart for a high-tier tool upgrade.
Navigation Shortcuts for the Weary
If the Putrified Ducts sound like a nightmare, there is a secondary route, but it’s arguably worse. You can come up from Bilewater after defeating Groal the Great.
I wouldn't recommend it.
Bilewater is infested with Muckmaggots, and the water is contaminated. Unless you’re wearing the Wreath of Purity (a tool that mitigates environmental damage), you’ll probably die before you even see a save bench. Stick to the Memorium route. It’s safer, even with the bats.
Summary of Apostate Key Locations
| Location | Purpose |
|---|---|
| Putrified Ducts | Where the key is actually found (far left side, breakable wall). |
| Lower Slab | Unlocks a shortcut to Choral Chambers. |
| Upper Slab | Unlocks a Mask Shard and The First Sinner boss room. |
Actionable Next Steps
If you’re currently staring at an Apostate lock and feeling the FOMO, here is exactly what you should do:
- Leave the Slab. If you've got your needle back, find the Bellway and get out. You aren't meant to finish this area yet.
- Push to Mount Fay. You need the Faydown Cloak. Without it, you can't reach the hidden ceiling in Memorium.
- Check your Tool Pouch. Ensure you have the Wreath of Purity if you decide to take the Bilewater route, though again, it's the "hard mode" path.
- Mark your map. Use one of the pin markers on the Choral Chambers/Slab border so you remember to come back once you’ve reached Act 2.
The Silksong the slab apostate key is a classic Metroidvania "check" to make sure you're exploring the wider world rather than just tunnel-visioning on the nearest locked door. It’s annoying, sure, but the lore and the Mask Shard at the end make it a mandatory stop for anyone trying to actually finish the game's tougher Act 3 challenges later on.
Go explore the Citadel. The key will be waiting for you when you're actually strong enough to use it.