The Hammer is the titular weapon in Dwarf Hammer, a mythical artifact designed to bludgeon God. Whether the various dwarfs who journey into the Caverns of Creation below Eden have different hammers, or if the same hammer somehow returns to the dwarf colony upon the death of its current champion, is unknown.
The hammer is the player's starting weapon, dealing at base 5 points of damage (with a +2 bonus for each level gained, and dealing double damage on top of the usual bonus for flesh creatures). In order to win the game, the player must use the hammer to deliver the killing blow to God. This is important, so we're going to say it again: in order to win the game, the player must use the hammer to deliver the killing blow to God.
Because of this, if the hammer is destroyed, dropped, traded, sold, or stolen, the game becomes unwinnable.