This key is given to Twinsen by Queen Astrid. It opens a passage behind her throne, that leads to the Island CX.
After meeting the queen, she promises to give the key to Twinsen if he passes the test of truth, but after he passes it, the island is invaded by the emperor's forces and the queen gets captured. To get the key, Twinsen has to find her in a cell in the Building Company.
Inventory Description[]
"This key will allow you to open the doors of the passage, whose entrance is in the village of the Mosquibees, and which will allow you to go up to islet CX."