Damage

Damage in NetHack reduces the current hit points of the monster or character that receives it. If a creature is reduced to 0 or fewer HP, he, she, or it dies. Exception: if you are a monster (due to polymorph or lycanthropy) and you're reduced to 0 or fewer HP, you revert to your original form.

Damage can come from a variety of sources, including physical and magical attacks, as well as falling down stairs or onto a sink, stepping on a trap, being hit by a bounced or reflected spell, and many others.

The physical damage you do can be increased by wearing a ring of increase damage, and decreased by erosion of the weapon you are using.

Damage from some spells and effects can be reduced or eliminated by having the appropriate resistance, magic cancellation, or magic resistance, or by having the half physical damage or half spell damage intrinsics.

The damage that a weapon, attack, or other occurrence can do is often specified in D notation, such as 3d6 meaning the sum of three 6-sided dice.