The Guardian Devil

The Guardian Devil helmet is one of the three possible rewards, other than Echo of the Fiend or Tongue of Acid by answering " murder " in Thrix's riddles in Avernus Bridge.

In-game description
"This black iron helm has a rough, spiked texture on the outside and a fanged devil's head engraved above the brow. Despite its uncomfortable appearance, its padded interior comfortably fits its wearer's head as though it was designed for it. Not only does the helm protect the wearer from critical hits, but it extends its protection to the rest of the wearer's body."