Library ninjutsu is a variant of ninjutsu that can be activated only from your library. It can’t be activated from your hand. Just as with regular ninjutsu, the Ninja enters attacking the player, planeswalker, or battle that the returned creature was attacking.

Library ninjutsu ability can be activated only while you’re searching your own library and only if you control at least one unblocked attacker. The effect that caused you to search needs to say “search” and “library”. Examples of effects that let you activate library ninjutsu are Primal Growth and Buried Alive. Examples of effects that don’t are Bribery and Halimar Depths.

Activating a library ninjutsu ability while searching your library follows all the normal rules for activating an ability, except for timing (activating this ability always occurs during the resolution of another spell or ability). The ability goes on the stack. You have to pay the ability’s listed costs and any applicable additional costs, which means you can activate mana abilities while you’re activating a library ninjutsu ability even as you’re searching your library.

After you activate a library ninjutsu ability, you continue the search effect where you left off. Once the search effect finishes resolving, the active player gets priority with the library ninjutsu ability on the stack. Any abilities that triggered when the ability was activated are put on the stack now.

If you have enough mana and unblocked attackers, you can activate the library ninjutsu abilities of multiple Panglacial Shinobi during a single search effect.

If you want to activate a library ninjutsu ability while searching your library, you must do so before you find any cards with the search effect.

While searching your library, you must keep your library in the same order until you shuffle it. This order could matter if you tap Millikin for mana, for example, to pay for a library ninjutsu ability.

Ninjutsu abilities can be activated only after blockers have been declared. Before then, attacking creatures are neither blocked nor unblocked.

As you activate a library ninjutsu ability, you reveal the Ninja card in your library and shuffle the attacking creature into your library. The Ninja card with library ninjutsu stays revealed and isn’t put onto the battlefield until the ability resolves or otherwise leaves the stack. If it leaves your library before then, it won’t enter the battlefield at all.

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 library 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 combat damage step or end of combat step, it will be after combat damage has been dealt, so the Ninja won’t deal combat damage.

If a creature in combat has first strike or double strike, you can activate the library 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.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.