If an opponent plays Possession (which targets an enemy unit at a battlefield), can you react with a spell that moves that unit to base, and would that make the target invalid?

Ruling: If you react to Possession by moving the targeted unit from a battlefield to your base, the target becomes invalid and Possession does not resolve.

Sequence:

• Opponent plays Possession, paying 8 energy and 3 purple power immediately
• Opponent must choose and target an enemy unit at a battlefield when playing the card
• You can then react, knowing which unit was chosen
• If you move the targeted unit to base with a reaction spell, the target becomes invalid
• Possession fails to resolve because the unit is no longer at a battlefield

Nuances:

• The targeting restriction "at a battlefield" applies both when choosing the target and when the spell resolves
• If the unit moved to a different battlefield instead of base, it would still be a valid target because it remains at a battlefield
• You must pay all costs (8 energy and 3 purple power) at the moment of playing Possession, not separately