Trogskull Helmet

Chest loot hoarded by a trogoldyte hiding in a secret room. Once you enter the room with the drawbridge, the valve to open the secret room is to the left, tucked away behind some crates. There are various ways to wiggle over, but it may be easier to lower the drawbridge first. This will give you a better vantage point to see the crates and the valve, and will put you in a better position to jump onto a narrow ledge in front of the crates so you can move past them.

Once you have activated the valve, the secret room will open on the other side of the drawbridge. As with most items like this, it will not always be present in the chest, and the troglodyte may not even spawn.