Damage

Damage is impairment or destruction that a creature, planeswalker, or player may suffer from a certain source.
 * 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.
 * Damage dealt to a Planeswalker causes that many loyalty counters to be removed from it.
 * Damage dealt to a player causes them to lose that much life.

While most damage is caused by the combat between creatures, or creatures attacking players, there are also many cards which can deal direct damage to creatures, planeswalker and/or players. These cards are usually red, e.g. Lightning Bolt.

Rules
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