Echo of the Fiend

On the far side of Avernus Bridge in front of the massive door to the Interior where the elevator leads the party up to face the final boss of, Thrix the gatekeeper will demand answers to his riddles before the party may venture forth, answer " murder " and he'll let you choose among Echo of the Fiend, The Guardian Devil, or a Tongue of Acid.

In-game description
"A devil of Avernus once possessed this gnarled wooden staff. Though the devil long since escaped from the staff, a residue of its essence permeated the weapon and lingers still. When a command word is spoken, an aura of fire springs up around anyone wielding the staff; though the flames leave the wielder untouched, they sear others with devastating heat."