The first Grond appears in The Silmarillion. In the First Age, Grond was the great mace of Morgoth Bauglír, the first Dark Lord, who wielded it when he fought with Fingolfin, High King of the Noldor Elves. With every blow from the mace that struck the ground, a crater was formed. Morgoth managed to kill the High King, but not without receiving seven wounds himself.
The second Grond appears in The Lord of the Rings. During the The Siege of Gondor in the Third Age, a great battering ram was named after the mace. A hundred feet long with a head shaped like a snarling wolf, it was used in the siege of Minas Tirith, pulled by "great beasts" (probably Oliphaunts, called that just a few sentences earlier) and operated by mountain-trolls. Aided by spells laid on it by the Witch-king of Angmar, lord of the Ringwraiths, and the spells cast upon it during its forging in Mordor, Grond destroyed the formidable gate of Minas Tirith in just three blows. After it had been used, it was pulled back so the forces of Mordor could enter the city. After this nothing more is known about it.
In some illustrations, the phrase "Hammer of the Underworld" is taken literally and Grond is erroneously depicted as a war hammer.
During the Siege of Minas Tirith, Gothmog orders for his soldiers to "Bring forth the Wolf's Head," in response to one of his captains stating that nothing can break the gates. Then, Grond (the ram) is brought out by the great beasts.