Blade-Blizzard Kitsune

If a creature in combat has first strike or double strike, you can activate the ninjutsu ability during the first-strike combat damage step. The Ninja will deal combat damage during the regular combat damage step, even if it has first strike.

As you activate a ninjutsu ability, you reveal the Ninja card in your hand and return the attacking creature. The Ninja card stays revealed and isn't put onto the battlefield until the ability resolves. If it leaves your hand before then, it won't enter the battlefield at all.

The ninjutsu ability can be activated during the declare blockers step, combat damage step, or end of combat step. In most cases (see below), if you wait until the combatdamage step or end of combat step, it will be after combat damage has been dealt, so the Ninja won't deal combat damage.