Ninjutsu

Ninjutsu is a keyword ability designed by Mark Rosewater that was introduced in Betrayers of Kamigawa. It is exclusive to Ninja creatures.

Description
Ninjutsu represents the stealth of the Ninjas by allowing a creature that attacks and isn't blocked to be exchanged with a Ninja in the controlling player's hand, such that the Ninja is attacking and unblocked instead of the original creature, which is returned to the owner's hand.

To incentivize this gameplay, all Ninjas printed in Betrayers of Kamigawa had a saboteur ability, making for more interesting payoffs rather than putting a large creature in cheaply. Sakashima's Student played in slightly different space as a clone, while in Modern Horizons Azra Smokeshaper used an enters-play ability that varied combat situations. Ninja of the New Moon went for the "large creature" approach, being otherwise virtual vanilla.

History
The keyword has only appeared in Standard during Betrayers of Kamigawa, and exclusively on black or blue Ninja cards. It first returned on some cards in the Night of the Ninja deck of Planechase 2012, then later it reappeared in Modern Horizons.

Disguise
A variant called Disguise was considered as the Dimir mechanic for Guilds of Ravnica. In this variant, the creature with disguise swaps with the attacking creature, meaning that anything connected to it (enchantments enchanting it, equipment equipped to it, counters on it, etc.) are still attached to the new creature as it was treated as the same object.

Zareth San, the Trickster
A similar ability can be found in the Zendikar Rising set on Zareth San, the Trickster. While the two abilities are mostly the same, there are a couple of differences between Zareth San's ability and ninjustu. Zareth San requires an unblocked Rogue, ninjutsu does not specify a creature type. Additionally, Zareth San can be attacking a different player/planeswalker than the one that the returned creature was attacking, which is not case for the creature put onto the field by the ninjustu ability.

Commander ninjutsu
undefined Commander ninjutsu is a variant of ninjutsu which works from both the hand and the command zone. This was featured on Yuriko, the Tiger's Shadow in Commander 2018.

Rulings

 * The ninjutsu ability can be activated during the declare blockers, combat damage, or end of combat steps if you have an unblocked attacking creature.
 * Activating the ninjutsu ability during the declare blockers step allows the Ninja to deal combat damage. Using ninjutsu during the combat damage or end of combat steps will mean the unblocked attacking creature is dealing combat damage.
 * The Ninja will be attacking the same player or planeswalker the original attacking creature was attacking. This is a rule specific to ninjutsu; in other cases, the controller of a creature put onto the battlefield attacking chooses which player or planeswalker it attacks.
 * Although the Ninja is attacking, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example).
 * The Ninja should be revealed while announcing the ability, but it doesn't leave your hand until the ability resolves.