Damage is destruction a creature, Planeswalker, or Player may suffer.
Damage dealt to a Planeswalker causes that many loyalty counters to be removed from it. Damage dealt to a player causes him or her to lose that much life. Damage dealt to creatures is removed from the creature at the end of each turn, unless the total damage dealt to that creature over the course of the turn equals or exceeds its toughness, which causes the creature to be destroyed and put into the graveyard unless another effect replaces this. An amount of damage larger than or equal to the toughness of a creature is called lethal damage.
While most damage is caused by the combat between creatures, there are many cards which can deal damage directly to creatures or players. These cards are usually red, e.g. Lightning Bolt. Damage to a player may be redirected to a Planeswalker that player controls provided that the player who controls the source of damage is an opponent of the player who controls the Planeswalker.
From the Comprehensive Rules (Aether Revolt (January 20, 2017))
- 119. Damage
- 119.1. Objects can deal damage to creatures, planeswalkers, and players. This is generally detrimental to the object or player that receives that damage. An object that deals damage is the source of that damage.
- 119.1a Damage can’t be dealt to an object that’s neither a creature nor a planeswalker.
- 119.2. Any object can deal damage.
- 119.2a Damage may be dealt as a result of combat. Each attacking and blocking creature deals combat damage equal to its power during the combat damage step.
- 119.2b Damage may be dealt as an effect of a spell or ability. The spell or ability will specify which object deals that damage.
- 119.3. Damage may have one or more of the following results, depending on whether the recipient of the damage is a player or permanent, the characteristics of the damage’s source, and the characteristics of the damage’s recipient (if it’s a permanent).
- 119.3a Damage dealt to a player by a source without infect causes that player to lose that much life.
- 119.3b Damage dealt to a player by a source with infect causes that player to get that many poison counters.
- 119.3c Damage dealt to a planeswalker causes that many loyalty counters to be removed from that planeswalker.
- 119.3d Damage dealt to a creature by a source with wither and/or infect causes that many -1/-1 counters to be put on that creature.
- 119.3e Damage dealt to a creature by a source with neither wither nor infect causes that much damage to be marked on that creature.
- 119.3f Damage dealt by a source with lifelink causes that source’s controller to gain that much life, in addition to the damage’s other results.
- 119.4. Damage is processed in a three-part sequence.
- 119.4a First, damage is dealt, as modified by replacement and prevention effects that interact with damage. (See rule 614, “Replacement Effects,” and rule 615, “Prevention Effects.”) Abilities that trigger when damage is dealt trigger now and wait to be put on the stack.
- 119.4b Next, damage that’s been dealt is processed into its results, as modified by replacement effects that interact with those results (such as life loss or counters).
- 119.4c Finally, the damage event occurs.
Example: A player who controls Boon Reflection, an enchantment that says “If you would gain life, you gain twice that much life instead,” attacks with a 3/3 creature with wither and lifelink. It’s blocked by a 2/2 creature, and the defending player casts a spell that prevents the next 2 damage that would be dealt to the blocking creature. The damage event starts out as [3 damage is dealt to the 2/2 creature, 2 damage is dealt to the 3/3 creature]. The prevention effect is applied, so the damage event becomes [1 damage is dealt to the 2/2 creature, 2 damage is dealt to the 3/3 creature]. That’s processed into its results, so the damage event is now [one -1/-1 counter is put on the 2/2 creature, the active player gains 1 life, 2 damage is marked on the 3/3 creature]. Boon Reflection’s effect is applied, so the damage event becomes [one -1/-1 counter is put on the 2/2 creature, the active player gains 2 life, 2 damage is marked on the 3/3 creature]. Then the damage event occurs.
Example: The defending player controls a creature and Worship, an enchantment that says “If you control a creature, damage that would reduce your life total to less than 1 reduces it to 1 instead.” That player is at 2 life, and is being attacked by two unblocked 5/5 creatures. The player casts Awe Strike, which says “The next time target creature would deal damage this turn, prevent that damage. You gain life equal to the damage prevented this way,” targeting one of the attackers. The damage event starts out as [10 damage is dealt to the defending player]. Awe Strike’s effect is applied, so the damage event becomes [5 damage is dealt to the defending player, the defending player gains 5 life]. That’s processed into its results, so the damage event is now [the defending player loses 5 life, the defending player gains 5 life]. Worship’s effect sees that the damage event would not reduce the player’s life total to less than 1, so Worship’s effect is not applied. Then the damage event occurs.
- 119.5. Damage dealt to a creature or planeswalker doesn’t destroy it. Likewise, the source of that damage doesn’t destroy it. Rather, state-based actions may destroy a creature or planeswalker, or otherwise put it into its owner’s graveyard, due to the results of the damage dealt to that permanent. See rule 704.
Example: A player casts Lightning Bolt, an instant that says “Lightning Bolt deals 3 damage to target creature or player,” targeting a 2/2 creature. After Lightning Bolt deals 3 damage to that creature, the creature is destroyed as a state-based action. Neither Lightning Bolt nor the damage dealt by Lightning Bolt destroyed that creature.
- 119.6. Damage marked on a creature remains until the cleanup step, even if that permanent stops being a creature. If the total damage marked on a creature is greater than or equal to its toughness, that creature has been dealt lethal damage and is destroyed as a state-based action (see rule 704). All damage marked on a permanent is removed when it regenerates (see rule 701.13, “Regenerate”) and during the cleanup step (see rule 514.2).
- 119.7. The source of damage is the object that dealt it. If an effect requires a player to choose a source of damage, he or she may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a prevention or replacement effect that’s waiting to apply, or by a delayed triggered ability that’s waiting to trigger (even if that object is no longer in the zone it used to be in); or a face-up object in the command zone. A source doesn’t need to be capable of dealing damage to be a legal choice. See rule 609.7, “Sources of Damage.”
- 119.8. If a source would deal 0 damage, it does not deal damage at all. That means abilities that trigger on damage being dealt won’t trigger. It also means that replacement effects that would increase the damage dealt by that source, or would have that source deal that damage to a different object or player, have no event to replace, so they have no effect.