Can I activate ninjutsu between first strike and normal damage steps

Question

Say I control a creature with first strike, that does not get blocked when attacks are declared.

Can I have it deal the first strike damage before activating/resolving the keyword ability ninjutsu on a ninja card in hand?

Answer 1

The comprehensive rules say about ninjutsu that, among other things,

702.48c A ninjutsu ability may be activated only while a creature on the battlefield is unblocked (see rule 509.1h). The creature with ninjutsu is put onto the battlefield unblocked. It will be attacking the same player or planeswalker as the creature that was returned to its owner’s hand.

Since it's referencing 509.1h, which happens to be in the section on the declare blockers step, let's see what that rule says:

509.1h An attacking creature with one or more creatures declared as blockers for it becomes a blocked creature; one with no creatures declared as blockers for it becomes an unblocked creature. This remains unchanged until the creature is removed from combat, an effect says that it becomes blocked or unblocked, or the combat phase ends, whichever comes first. A creature remains blocked even if all the creatures blocking it are removed from combat.

So yes, a creature which is attacking and unblocked stays that way until combat is over (which includes a step after all damage is dealt but before your second main phase), unless something happens to it. Note that it is only an unblocked creature once blockers have been declared. You can even ninjutsu out your ninja after regular combat damage has been dealt. Or, as you say you want to do, after your attacking, unblocked creature has dealt its first strike damage, but before regular combat damage is dealt, for an effective "double strike".

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There is a funny subtlety here, though. If you, for some reason, have a ninjutsu creature with first strike on your hand, and there are no creatures with first strike in combat, and you want to ninjutsu out your creature so that no damage is dealt to the defending player neither by your attacking unblocked creature nor by your ninjutsu creature, then that cannot be done. That's because if there are no creatures with first strike or double strike in combat as the declare blockers step ends, then there is no first strike damage step. So either you ninjitsu out your creature after blockers are declared, and you get a first strike step, or you wait, which means the game goes straight to the regular damage step, where the unblocked attacking creature deals its damage.

And even if there is a creature with first strike in combat (say one of your other creatures that got blocked), so you do get a first strike step, it wouldn't help, as who deals damage in the second damage step actually doesn't really care whether creatures have first strike. In that step, damage is dealt by creatures that have double strike, as well as creatures that didn't deal damage in the previous step. So if you ninjutsu after first strike damage is dealt, then your ninjutsu creature with first strike deals regular combat damage.

There are more important applications of this subtlety, though, and what I suspect is the reason behind why the regular damage step doesn't care about first strike directly: giving first strike to an opponent's creature with the right timing doesn't prevent it from dealing its combat damage. Similarly, there are a few instances where a creature could lose first strike. But no matter how you try to time it, that would not give it an opportunity to double strike.

Back to your situation, it would also mean you would get to deal damage with both creatures even if the ninjutsu creature had first strike. I can't find any such creatures in Gatherer, but an effect like "creatures you control have first strike", of which there are several, would actually make this situation happen.

Answer 2

Yes, you can do that

Ninjitsu can be activated after the first combat step.

The ninja that just came into play will even still do damage in the second combat step. Even if the new ninja had first strike because of following rule:

510.5. If at least one attacking or blocking creature has first strike (see rule 702.7) or double strike (see rule 702.4) as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that had neither first strike nor double strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.

Answer 3

I wasn't able to find extensive explanations on the Ninjutsu ruling, but I did find something:

From MTG gamepedia, rulings section (emphasis mine):

The ninjutsu ability can be activated during the declare blockers, combat damage, or end of combat steps if you have an unblocked attacking creature.

which is supported by rule 702.48c:

A ninjutsu ability may be activated only while a creature on the battlefield is unblocked (see rule 509.1h). The creature wit h ninjutsu is put onto the battlefield unblocked. It will be attacking the same player or planeswalker as the creature that was returned to its owner’s hand.

Also, rule 510.4 states that:

If at least one attacking or blocking creature has first strike (see rule 702.7) or double strike (see rule 702.4) as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that had neither first strike nor double strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.

Given those rulings, Ninjutsu can be activated during the first strike damage phase, if you hold priority before proceeding to the normal damage step.