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. However, each time the Maw is used, it burns away a part of the wielder's soul. On average, the wielder would be able to use the Maw three or four times before getting sent to Hell. A Darkness host like Jackie could theoretically use the Maw as much as desired, due to the host's soul already being destined to Hell.
The Abysmal Maw now doesn't 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.