When Player A plays Stand United from hidden at battlefield A targeting their unit, and Player B plays Mystic Reversal to gain control of it: (1) Does Stand United retain the targeting restriction to the battlefield it was hidden from? (2) If Player B controls no units at battlefield A, how does resolution proceed?

Ruling: When a spell played from hidden is controlled by another player via Mystic Reversal, it retains the targeting restriction to the battlefield it was played from. The new controller cannot make new choices for targets, and if the original target is no longer friendly to them, the buff portion fails but other effects (like "buffs grant +1 Might this turn") still apply.

Sequence:

• Stand United is played from hidden at battlefield A with targeting restriction "here"
• Player B gains control via Mystic Reversal
• The spell keeps its original target (cannot be changed)
• On resolution, the buff fails because the target is not friendly to Player B
• Player B still gets the "buffs grant +1 Might this turn" effect

Nuances:

• Gaining control of a spell does not allow replaying it or changing its state - it was already finalized to the chain
• The original target becomes illegal for the new controller because it's not a friendly unit to them
• Secondary effects of the spell that don't depend on the illegal target still resolve for the new controller