If Hidden Blade is flipped from hidden to kill a unit at a battlefield, and in response Tideturner is flipped to move that unit to a different battlefield, does Hidden Blade lose its target?

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Accepted Answer

Yes. If you flip Hidden Blade from Hidden and target a unit at the battlefield where it was hidden, then your opponent flips Tideturner and uses its play effect to move that unit to a different battlefield while Hidden Blade is on the chain, the unit ceases to be a legal target for Hidden Blade (so it won't be killed). Hidden Blade's draw effect still occurs.

When Hidden Blade is played from Hidden it must choose its target from the battlefield where it was hidden. If Tideturner's play effect moves that targeted unit to a different battlefield while Hidden Blade is on the chain, that unit is no longer a legal target for Hidden Blade and will not be killed when Hidden Blade resolves. Any non-targeted parts of the spell (the controller-draws-2 instruction) still happen as applicable.

You flip Hidden Blade from Hidden; as you put the spell on the chain you choose a unit that is at the battlefield where Hidden Blade was hidden as the spell's target [355.1] [811.1.d.2]. An opponent flips Tideturner from Hidden and plays it as a Reaction (Hidden grants Reaction) and chooses that unit as the target of Tideturner's play effect; Tideturner's play effect moves the two units, putting the originally targeted unit onto a different battlefield [811.6] [811.1.d.2]. While Hidden Blade is still on the chain, its targeted unit is no longer at the battlefield where Hidden Blade was played; that target ceases to be a legal target for Hidden Blade because the spell played from Hidden may only target things at its battlefield [359.3.e.4]. When Hidden Blade resolves the kill instruction can't affect that unit (it is unaffected), but the draw instruction still occurs for that unit's controller if the spell's other parts remain valid [359.3.e.5] [359.3.e.6].

If Tideturner moved the unit to a non-board zone (trash, banished, etc.), the unit would no longer be the same game object and the targeting relationship would be severed; it would definitely be an illegal target and unaffected [359.3.e.4]. If Tideturner moved the unit back to the original battlefield before Hidden Blade finished resolving, the unit would be a legal target again and could be affected (targets can regain legality if they meet requirements again) [359.3.e.3]. If Hidden Blade had been played normally (not from Hidden), its targeting options might differ; the Hidden-specific battlefield restriction is what causes loss of legality when the unit moves away [811.1.d.2]. If multiple units were targeted by a multi-target spell and some become illegal while others remain legal, the spell can still affect the legal targets and the illegal ones are simply unaffected; the spell's other instructions still resolve as applicable [359.3.e.5] [355.11].

When a card with Hidden is played from Hidden, any targets it chooses must be chosen from among options at the battlefield where it was hidden; for spells that target this imposes a battlefield-specific targeting restriction as part of putting the spell on the chain [811.1.d.2]. Choices for a spell are made when the spell is played/finalized on the chain [355.1]. Once Hidden Blade is on the chain, it remembers the unit it targeted. However, a target can cease to be a legal target if it moves away from the battlefield where the Hidden spell was played; a spell played from Hidden can normally only target its own battlefield or something at that battlefield, and a target that leaves that battlefield becomes illegal for that spell [359.3.e.4]. The example and rules make clear that moving a targeted unit to a different battlefield while the spell is on the chain causes it to be no longer a legal target for that Hidden spell. When a spell resolves and one of its targets is no longer legal, that target is unaffected by the spell, while other parts of the spell (for example, draw instructions) still occur as written [359.3.e.5] [359.3.e.6]. Also note that cards hidden facedown gain Reaction and thus may be played in response as Tideturner was here [811.6]. Therefore, if Tideturner moves the targeted unit to a different battlefield after Hidden Blade was played from Hidden, Hidden Blade will not be able to kill that unit when it resolves; however, the draw-2 instruction still happens for that unit's controller.