This key, wich is shaped like a pyramid, is the key to the secret room that contains Sendell's Ball, in the sewers of Citadel Island.
After bringing the gallic acid to Twinsen's neighbour, he uses it to restore some documents belonging to his late son. This reveals a map with instructions that mention the key, which the neighbour then gives to Twinsen.
Inventory Description[]
"This Pyramid is a key. If you put it in the right place, it will activate a mechanism giving you access to a secret place."