Like the ordinary aspergillum, the heavy aspergillum took the form of a metal club or mace, with a reservoir inside that could contain up to three flask's worth of holy water. Pressing a button on the shaft opened up the perforations in the head, allowing holy water to leak out. In this way, the aspergillum could spray holy water over a short distance or on anything struck by it, using one flask's worth per shot.
Unlike the ordinary aspergillum, the heavy aspergillum was designed as a melee weapon, with the effectiveness of a heavy mace.
In addition to its ritual uses, the heavy aspergillum could be used as a weapon as well as a means of spraying monsters with holy water. Circa 1372 DR, it was used almost exclusively by the worshipers of Ilmater the Crying God in the kingdom of Impiltur against the demons that terrorized that realm.