Bedeviling Stranger

When Bedeviling Stranger is played from face down, its play effect can choose only the battlefield it was played to and a legal enemy unit there; when played from anywhere else, it can choose any battlefield containing an enemy unit it can legally target.

The play effect has two targets: the battlefield it tells you to choose and the enemy unit mentioned in its later return instruction.^355.10 You choose both targets while the ability finalizes, even though you decide whether to return the enemy unit later during resolution.^{383.3.a.3403.2} When a permanent with [Hidden] is played from face down, each target of its play effect must be chosen from options at the battlefield that permanent was played to unless the ability's own targeting restriction makes this impossible.^811.1.d.3 A battlefield is itself a game object at its own location, so only the battlefield where Bedeviling Stranger was played satisfies this restriction.^199.1 If no legal enemy unit is there, the pending play effect is removed from the chain without finalizing.^403.4

This targeting restriction applies only when a card is played from face down.^811.1.d Playing Bedeviling Stranger from your hand, or from your trash through Last Rites or Endless Riches, therefore lets you choose any battlefield with a legal enemy unit there.

Bedeviling Stranger checks for four or more units at the chosen battlefield only when its ability resolves, not during finalization. You choose both the battlefield and an enemy unit there as targets during finalization, even if fewer than four units are there then.^355.10403.2 The later "if there are 4 or more units there" clause is part of the effect because it does not immediately follow the "When you play me" trigger condition.^{383.2.a.1383.2.b} An instruction with no other stated timing executes during resolution.^135.2.b.3

After the ability finalizes on the chain, players can play cards and abilities with [Reaction] before it resolves.^407.4 When the ability resolves, it rechecks whether the same targets still meet every targeting restriction.^{359.3.e.2359.3.e.2.a407.5} If those responses leave fewer than four units at the chosen battlefield, the ability still resolves, but its return instruction does not execute.^{135.2.b.5.b407.5}

If Bedeviling Stranger was played from face down and leaves the board after its play effect finalizes but before it resolves, the effect still resolves, but both targets become illegal because the missing source cannot identify the battlefield required by [Hidden], so its return instruction is ignored.^{359.3.e.5359.3.e.6407.5811.1.d.3.a} The finalized ability remains on the chain after its source leaves.^407.2

The target choices themselves do not change.^355.15 Instead, the ability rechecks whether those targets still satisfy every targeting restriction, including restrictions imposed by other effects.^{359.3.e.2359.3.e.2.a} Both targets depend on the battlefield information supplied by Bedeviling Stranger as the source of an effect restricted by [Hidden].^{811.1.d.3811.1.d.3.a}

An opponent can create this outcome by responding to the finalized play effect with Gust or Star-Crossed.^407.4 The newer spell resolves first and returns Bedeviling Stranger to its owner's hand, so the play effect resolves afterward without returning an enemy unit.^340.1