When the wielder holds the Abysmal Maw on his right hand and recites a certain incantation, the Maw will unleash a blast of pure Hellfire, destroying everything in its trajectory; anyone killed by the blast will get sent to Hell, regardless of his virtues. However, each time the Maw is used, it burns away a part of the wielder's soul. On average, the wielder could use the Maw three or four times before getting sent to Hell. A Darkness host like Jackie could theoretically use the Maw ad nauseam, due to the host's soul already being connected to Hell. The Brotherhood created a special glove called the Deceiver's Grasp to protect users of the Maw by condemning another to Hell in their stead.
The Abysmal Maw now does not work anymore since the incantation is lost to time. Johnny Powell muses that the Brotherhood is to blame since they were the last group to use the Maw, perhaps during World War II, but this also means that there is a Brother who might remember the incantation.
The relic is found in the Hellgate Field in a side room from which more Brotherhood enforcers burst out, during the ambush.