Before leaving the island in act one, I went back to the vault with the cursed lever. To my surprise, I found another tyrant's helm.
I had already defeated the demon a while back ago, so I don't know how long it takes to re-spawn, but now I have two helmets. A "Tyrant's Helm" with the Shackles of Pain ability, and a "Corrupted Tyrant's Helm" with the devour ability.
Note: The demon doesn't appear again after using the devour ability multiple times, so no experience exploit.
