The mauler was a humongous weapon that was originally developed by the Romulans and later sold to the Klingons and Lyrans. The mauler is an immense beam weapon, so large that a ship had to be specially modified to carry it and has little room for anything else. Most of the ship was filled with just the mauler cannon and banks of batteries to power it. Although the mauler was capable of inflicting more damage than any other weapon in the Star Fleet Battles Universe, its use was generally limited to special missions where that kind of firepower is needed, like starbase assaults. Maulers rarely operated alone because they carry few (if any) other weapons, and if the mauler cannon itself were fired too often, the stresses involved would eventually tear the ship apart. (Star Fleet Battles)
It was once said that the mauler was equipped with a starship. (TOS video game: Starfleet Command 2: Orion Pirates)
Maulers were used to some effect during the Battle at the Grave of Thoughts, both to destroy Kzinti dreadnoughts and to crack the shields of the Grave of Thoughts. (RIS Bouteina: Battle at the Grave of Thoughts, Redshirt Distribution)