Bushido is a triggered ability that rewards creatures in combat. A creature with bushido N gets +N/+N until end of turn whenever it blocks or becomes blocked. For example, a creature with bushido 2 gets +2/+2 in either situation, while bushido 5 grants +5/+5. The bonus lasts only for that turn, and it triggers each time the creature blocks or becomes blocked. A creature that also has double strike still gets the bushido bonus whenever the condition is met.
In play, bushido makes attacking and blocking more dangerous for the opponent. A blocker with bushido can suddenly deal extra damage, so attacks become less profitable, while an attacker with bushido punishes chump blocks or trades. Because the boost arrives after blocks are declared, it can create surprise damage and swing otherwise even combats. It is a combat-focused ability that makes every block a potential problem.
