Rule 510.2

Second, all combat damage that’s been assigned is dealt simultaneously. This turn-based action doesn’t use the stack. No player has the chance to cast spells or activate abilities between the time combat damage is assigned and the time it’s dealt. Example: Squadron Hawk (a 1/1 creature with flying) and Goblin Piker (a 2/1 creature) are attacking. Mogg Fanatic (a 1/1 creature with the ability “Sacrifice Mogg Fanatic: Mogg Fanatic deals 1 damage to any target.”) blocks the Goblin Piker. The defending player sacrifices Mogg Fanatic during the declare blockers step to deal 1 damage to the Squadron Hawk. The Hawk is destroyed. The Piker deals and is dealt no combat damage this turn. If the defending player instead left Mogg Fanatic on the battlefield, the Fanatic and the Piker would have dealt lethal damage to one another, but the Squadron Hawk couldn’t have been dealt damage.

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