The battering ram used to destroy the Great Gate of Minas Tirith, at the Battle of the Pelennor Fields on the 15th of March T.A. 3019.
The ram took its name from the ancient weapon of Morgoth, the Hammer of the Underworld. Morgoth bore this weapon in the battle with (and ultimately slaying of) Fingolfin, High King of the Noldor. Though it is mentioned as a weapon of mace-like character, the description of the battle and its use in it, suggests that is was shaped like a hammer. The battle at the gates of Angband is the only source of information on the weapon.
What became of the ancient weapon when Melkor was cast into the Void is unknown, but it is likely to have been lost in the War of Wrath at the end of the First Age, at the destruction of Angband and the imprisonment of Morgoth.
The name is ultimately derived from elvish, meaning "club".