The Blackest Vault | Salt and Sanctuary Wiki

Finding The Blackest Vault and the Order of the Betrayer Creed.

1. In order to find this creed, you will first need to have used a Stained Page on a sanctuary that is not of your creed, and died to the aggroed NPCs. When you die this way, you will obtain a Bloody Writ.
2. Once you have the writ, head to Siam Lake and go right from the sanctuary. You will need a torch!
3. Find a gap in the ground with water dripping down, then follow the water to fall down onto platforms below.
4. Be careful that the last drop is for a collapsing platform, so perform a dart jump at the end.
5. Past the collapsing platform stands The Gatewoman. Talk to her, hand over the Bloody Writ, and answer 'Yes' to her question to receive the Skull Trophy and the Bone Key.  A dart jump to the end of the collapsing platform, touching down, and then two dart jumps to the left yields a treasure chest with a Drowned Tome in it.  Return the same way, or warp out.

A video of how to do it below:

Full  The Blackest Vault Walkthrough

A full walkthrough including all the items, enemies and bosses is shown below. You can add links, images. and other details.

Open the locked door behind The Gatewoman with the Bone Key and go down the stairs until reaching the Sanctuary. Exit to the right and collect the Stone Guide before jumping onto the collapsing platform. Use the Vertigo and Dart Brands to reach a platform above with an obelisk. Activate the obelisk and continue to the right. The tenth jump will cancel the obelisk effect and allow the player to land on a platform to the left. The next room contains a Gray Pearl and sets of stairs that leads to Pitchwoods, just before Carsejaw the Cruel.

It's worth noting that it's technically possible to join the Order of the Betrayer, unlock the door, rejoin another creed and then convert the sanctuary to that creed. However, since this resets devotion with the original creed, and the sanctuary is inconveniently placed, doing so is not advisable.