The first thing that happens in the declare blockers step is that the defending player chooses which creatures will block and how they’re blocking. Once that happens, the blocks are locked in. Attackers that had a blocker declared become “blocked” creatures, and nothing can change that (aside from a few obscure effects that specifically say that a creature can become “unblocked,” none of which exist in Standard).

For example, you attack with Kruin Outlaw and a team of werewolves, and your opponent blocks each attacker with a 1/1 token. After blockers are declared, you cast Moonmist, which transforms all of your humans into their night side form. This means Kruin Outlaw is now Terror of Kruin Pass, which has an ability that says each werewolf you control can’t be blocked except by 2 or more creatures. While you might think this would cause the attackers to become unblocked, keep in mind that at the time blocks were declared, they were legal. The Terror’s ability only matters during the process of declaring blockers, and at that point, it was still Kruin Outlaw. The attacking creatures will remain blocked for the rest of combat, and no damage will get through to the player (unless an attacking creature has trample). The same is true if the blockers leave play during the declare blockers step.

Trample allows you to assign extra damage to the defending player (or planeswalker) after you assign lethal damage to the creature(s) blocking it. Note that if the blocking creature is no longer in combat (because it left the battlefield, perhaps) when assigning damage from a creature with trample, you must assign all the damage to the player or planeswalker. That’s because you can’t assign any combat damage to a creature that isn’t in combat.

Today’s Rules Tip written by
Ronny Alvarado, Level 1 Judge from Houston, TX