Slumping metal door
Slumping metal doors are three locked doors found in 
Bethesda Susa's troll lairs:
- A copper
slumping metal door, located in 
Jotun, Who Parts Limbs' lair, unlocked via a 
bronze key - A silver
slumping metal door, located in 
Fjorn-Kosef, Who Knits The Icy Lattice's lair, unlocked via a 
silver key - A gold
slumping metal door, located in 
Haggabah, Who Plies The Umbral Path's lair, unlocked via a 
gold key
Each troll's lair has a small room located at alternating southern corners containing 
stairs down, leading further into Bethesda Susa. The room is composed of 
marble walls; entry is permitted through a slumping metal door, unlocked via the corresponding key. One can always be found within the troll's inventory, though such a key can also be obtained in other ways, notably as a reward from village quests. The key can be retrieved by killing the troll, or by trading with him if the player's reputation with trolls is sufficiently high.
Entering the room containing the stairs does not necessarily require the player to go through the door; they can also break the walls or circumvent them through other methods, such as 
phasing through them or 
teleporting past them. Additionally, creating stairs down using a 
spiral borer can allow the player to skip the room entirely.