Example: A creature has the ability “Whenever this creature becomes blocked by a white creature, destroy that creature at end of combat.” If that creature is blocked by a black creature that is later turned white, the ability will not trigger.
310.1. As the combat damage step begins, the active player announces how each attacking creature will assign its combat damage. Then the defending player announces how each blocking creature will assign its combat damage. All assignments of combat damage go on the stack as a single object. Then any abilities that triggered on damage being assigned go on the stack. (See rule 410, “Handling Triggered Abilities.”) Then the active player gets priority and players may play spells and abilities.
310.2. A player may divide a creature’s combat damage as he or she chooses among the legal recipients. Dividing combat damage is subject to the following restrictions:
310.2a Each attacking creature and each blocking creature will assign combat damage equal to its power. Creatures with 0 or less power don’t assign combat damage.
310.2b An unblocked attacking creature will assign all its combat damage to the defending player.
310.2c A blocked creature will assign combat damage, divided as its controller chooses, to the creatures blocking it. If no creatures are currently blocking it (if, for example, they were destroyed or removed from combat), it will assign no combat damage.
310.2d A blocking creature will assign combat damage, divided as its controller chooses, to the attacking creatures it’s blocking. If it isn’t currently blocking any creatures (if, for example, they were destroyed or removed from combat), it will assign no combat damage.
310.2e An effect that states a creature deals its combat damage in a different manner than normal affects the assignment of combat damage.
310.3. Although combat-damage assignments go on the stack as an object, they aren’t spells or abilities, so they can’t be countered.
310.4. Combat damage resolves as an object on the stack. When it resolves, it’s all dealt at once, as originally assigned. After combat damage finishes resolving, the active player gets priority.
310.4a Combat damage is dealt as it was originally assigned even if the creature dealing damage is no longer in play, its power has changed, or the creature receiving damage has left combat.
310.4b The source of the combat damage is the creature as it currently exists, or as it most recently existed if it is no longer in play.
310.4c If a creature that was supposed to receive combat damage is no longer in play or is no longer a creature, the damage assigned to it isn’t dealt.
310.5. At the start of the combat damage step, if at least one attacking or blocking creature has first strike (see rule 502.2) or double strike (see rule 502.28), creatures without first strike or double strike don’t assign combat damage. Instead of proceeding to end of combat, the phase gets a second combat damage step to handle the remaining creatures. In the second combat damage step, surviving attackers and blockers that didn’t assign combat damage in the first step, plus any creatures with double strike, assign their combat damage.
311.1. As the end of combat step begins, all “at end of combat” abilities trigger and go on the stack. (See rule 410, “Handling Triggered Abilities.”) Then the active player gets priority and players may play spells and abilities.
311.2. As soon as the end of combat step ends, all creatures are removed from combat. After the end of combat step ends, the combat phase is over and the postcombat main phase begins.
312.1. The end phase consists of two steps: end of turn and cleanup.
313.1. As the end of turn step begins, all abilities that trigger “at end of turn” go on the stack. (See rule 410, “Handling Triggered Abilities.”) Then the active player gets priority and players may play spells and abilities.
313.2. If “at end of turn”-triggered abilities are created or if cards with “at end of turn”-triggered abilities come into play after preexisting ones have already gone on the stack at the beginning of the end of turn step, those abilities won’t go on the stack until the next turn’s end phase. In other words, the step doesn’t “back up” so new “at end of turn”-triggered abilities can go on the stack. This only applies to triggered abilities that say “at end of turn.” It doesn’t apply to continuous effects whose durations say “until end of turn” or “this turn.” (See rule 314, “Cleanup Step.”)
314.1. If the active player’s hand contains more cards than his or her maximum hand size (normally seven), he or she discards enough cards to reduce the hand size to that number (this game action doesn’t use the stack).
314.2. After discarding, the following actions happen simultaneously: all damage is removed from permanents and all “until end of turn” and “this turn” effects end (this game action doesn’t use the stack).
314.3. If the conditions for any state-based effects exist or if any triggered abilities are waiting to be put onto the stack, the active player gets priority and players may play spells and abilities. Once the stack is empty and all players pass, another cleanup step begins. Otherwise, no player receives priority and the step ends.
4. Spells, Abilities, and Effects
400.1. An ability is something an object does or can do. Abilities generate effects. An object’s abilities are defined in the object’s text box (if it has one) or by the effect that created the object. Abilities can also be granted to objects by effects. Reminder text and flavor text are not abilities. Reminder text and flavor text always appear in italics.
400.2. Spells, activated abilities, and triggered abilities generate effects when they resolve. Static abilities generate continuous effects. Text itself is never an effect.
401.1. A card on the stack is a spell. As the first step of being played, the card becomes a spell and goes on the stack from the zone it was played from (usually the player’s hand). (See rule 217.6, “Stack.”)
401.1a A copy of a spell is also a spell, even if it has no card associated with it.
401.1b A nonexistent mana cost can’t be paid.
401.2. A spell stops being a spell when it resolves (see rule 413, “Resolving Spells and Abilities”), is countered (see rule 414, “Countering Spells and Abilities”), or otherwise leaves the stack.