Nonlethal damage, also called subdual damage or striking to subdue, refers to a rule in Dungeons & Dragons which allows an attacker to knock an opponent out rather than kill them.
History[]
Original D&D[]
In Monsters & Treasure (1974), p.12-13, a rule specific to the dragon monster entry allows a dragon to be subdued and sold on the open market. The damage dealt to the dragon in one round is divided by its hit point total to determine the chance it will be subdued that round. The dragon takes no actual damage from a subdual attempt, and the concept of "nonlethal" points of damage did not exist in the original 1974 game.
AD&D 1st edition[]
The rule for non-lethally attacking creatures is known in this edition as striking to subdue, and appears in the Dungeon Masters Guide (1e) (1979), p.67. When fighting this way, all damage dealt is 75% temporary and 25% real. Certain creatures cannot be subdued, as defined in their monster entry, and player characters can never be subdued. The Monster Manual (1e) (1977), p.30 defines special rules for subduing, capturing and selling live dragons.
AD&D 2nd edition[]
Advanced Dungeons & Dragons 2nd edition introduces various rules under the heading Attacking Without Killing (Player's Handbook (2e) (1989), p.97-98, and Player's Handbook (2e revised) (1995), p.128-131). Non-lethal weapon attacks now impose a -4 penalty and deal 50% normal damage, of which 50% is temporary. Rules are also introduced for unarmed punching, wrestling, and overbearing.
Unique to this edition is a subjective limitation on which weapons may deal nonlethal damage: only bludgeoning weapons or those where an attacker can use the flat of a blade may be used nonlethally. Later editions would forgo this.
D&D 3rd edition[]
Dungeons & Dragons 3rd edition introduces subdual damage, renamed in D&D 3.5 to nonlethal damage (Player's Handbook (3.5) (2003), p.146). As in 2nd edition, dealing nonlethal damage with a lethal weapon imposes a -4 penalty, but now all damage dealt is nonlethal and there is no damage penalty. Nonlethal damage heals more quickly, and unarmed strikes deal nonlethal damage by default. Certain creatures are immune to nonlethal damage, most notably constructs, oozes and undead.
D&D 4th edition[]
In this edition, a much simpler non-lethal rule appear on Player's Handbook (4e) (2008), p.295. Whoever reduces a creature to zero hit points or below may opt to have it knocked unconscious instead of killed. There is otherwise no distinction between lethal and nonlethal damage in the game.
D&D 5th edition[]
D&D 5th edition has a similar nonlethal damage rule to 4th edition, appearing in the Player's Handbook (5e) (2014), p.198 ("Knocking a Creature Out"). An attacker who reduces a creature to zero hit points with a melee attack may choose to knock them out instead of kill them.