A a literal skeleton key carved from Azema's bones. The top is decorated in gold.

The key is capable of unlocking the crypt that leads to Azema's tomb underneath Sabek. It also is able to unlock most other doors throughout the city.

For every dark feeling we seek to hide
A deeper passion and love burns from the inside
Turning the keys and opening the lid
Both bones and treasure in such boxes are hid. - Symbols, Poems by Bertram Hoogna - Servants of Azema (GS-ADV2)

You may manage to acquire the key from a priest in the Temple of Ulos. He informs you to use it to find Yuvan Rokta in the catacombs of Sabek.

If you are not persuasive enough with the priest, you will instead come into possession of the key in the graveyard. Following Castien into the catacombs, he will forget to remove the key from the lock…but you do not forget.

The key truly does open more than just the entrance to the catacombs. It is also useful for:

• Opening a closed off section in the graveyard containing many members of the Council of Founders.
• Starting a musical tune at Glormax's tomb.
• Opening the vault in Gulpax's workshop, if the locks are unlocked in the correct sequence.
• Opening a number of locked chests.