Sanctum Key
Spoilers Ahead
This page contains spoilers for Post-Room 46 Gameplay. View at your own discretion.
Sanctum Keys are unique special keys in Blue Prince. Scattered throughout Mount Holly in various hidden locations, they are usually discovered around the same time Room 46 has been reached for the first time, and all serve a similar purpose.
How to obtain
There are a total of eight Sanctum Keys in the game. A poem hinting at the location of each of the eight Sanctum Keys can be found in Room 46, on Herbert S. Sinclair's desk.
The poem in question, depicting the eight Inner Sanctum doors below an Inneclipse symbol
Spoilers Ahead
The sections below contain details for the locations of all Sanctum Keys. View at your discretion.
#1
"The first key found here on the furthest rank"
Location: Room 46
The Sanctum Key is located on the desk, right next to the poem containing the hints.
This can only be collected the second time the room is visited, as the first time entering Room 46 immediately ends the day.
#2
"The second locked tight in the vault of a bank"
Location: Vault
The Sanctum Key is located in deposit box 370. Vault Key 370 is required to open the safe.
#3
"The third is high up, among all the clocks"
Location: Clock Tower
The Sanctum Key is the reward of the puzzle in the Clock Tower. See Clock Tower#Puzzle for more information.
Spoilers Within
This section contains spoilers for full puzzle answer.
To get the Sanctum Key, you must stand in the Clock Tower at the sacred hour to open a side room with 8 configurable clocks.
From left to right, these clocks should be set to 5:00, 5:08, 5:57, 6:00, 7:00, 8:00, 8:05, 9:00. Doing this causes a compartment under the fifth clock to open, containing the key.
#4
"And the fourth down below, beneath all the docks"
Location: Reservoir
Lowering the Reservoir's water level to 0 via the Pump Room allows the player to access the bottom level of the reservoir, which contains a large amount of trunks. The Sanctum Key is contained in one of them. See Reservoir#"Full Drain" for more information.
Spoilers Within
This section contains spoilers for exact location.
The Sanctum Key is inside the solitary black 
box tucked away on the back side of the pillar farthest from the stairs, outside the water.
#5
"The fifth key is held by a king on his throne"
#6
"The sixth in a station, left by one of your own"
Location: Safehouse
The Sanctum Key can be found on the same table that also contains the Safehouse Heist Plans.
#7
"The seventh is major if you draft it quite late"
Location: Music Room
This Sanctum Key spawns as the "major key" from the Music Room's effect if the room was drafted late, replacing the ordinary major key that would spawn in the room.
exact requirements
This Sanctum Key only spawns once Room 46 has been reached at least once.
Additionally, at least one of the following requirements must be fulfilled:
- The Music Room is drafted on Rank 9.
- At least 36 rooms were drafted today before the Music Room. (This includes the Outer Room and excludes any permanent rooms from a previous day.)
- The Music Room is drafted later than 7:05 PM in-game time (approximately 83 minutes from start of day).
#8
"And the last key is waiting, behind the last door of eight"
Location: Mechanarium
The Mechanarium can have up to eight doors, depending on the amount of 
floorplans in the manor at the time of drafting it. The Sanctum Key is behind the fourth additional door that does not lead to an adjacent room.
Unlike with other Sanctum Keys, this Sanctum Key's location has items that only spawn if the Sanctum Key is unavailable, that being 8
.
If a Sanctum Key is not used to unlock a door, e.g. because of insufficient steps, because the player does not yet know where to use it, or traded in the Trading Post, it must be acquired again. Sanctum Keys generally reappear in the same location on subsequent days until they have used unless they have been stored in the Coat Check or retained by the Moon Pendant.
Sanctum Key bug
There have been a number of reports of Sanctum Keys which have either failed to spawn when they seemingly should have, or have spawned twice from the same source on different days. All Sanctum Keys are essentially the same item and are only differentiated by their respective sources, each of which is controlled by a permanent variable or "mark" which determines whether or not the key should spawn. Ordinarily, any particular key spawns until it is used, at which point the source for that key is marked as used and can no longer be obtained.
It appears that, under certain circumstances, a Sanctum Key can be marked as obtained incorrectly, preventing it from spawning. This is frequently reported as one key being marked as obtained instead of another, meaning one key may be obtained twice and another key not at all. It is unclear how, why, or when this happens; it may occur when two keys are obtained on the same day, in which case it is possible that the game has "lost track" of which Sanctum Key is which. However, the bug has also been reported to occur when any key is obtained by itself and then lost the same day without using it.
In any case, it seems that when a Sanctum Key fails to spawn when it otherwise would, the most common reason is that a previously used Sanctum Key has begun spawning again. Checking the locations of previous Sanctum Keys frequently fixes the issue.
If you've found a new clue, you know what to do.
What causes the Sanctum Key bug?
Usage
Spoilers Within
This section contains spoilers for the Sanctum.
The Inner Sanctum contains 8 doors in the main area. Interacting with one of these doors while holding a Sanctum Key gives a prompt to permanently open the door using the key.
Once used, the Sanctum Key is lost (and no longer spawns in the location it was obtained) and the chosen door is permanently unlocked. All Sanctum Keys are interchangeable and the doors can be opened in any order.