If Void Seeker targets a unit at battlefield and the opponent responds by moving it to base with Flash, does the caster have to select a new target or kill their own unit?

Ruling: When a spell's target becomes illegal (such as moving from battlefield to base when the spell requires "a unit at a battlefield"), the spell does not select a new target. The illegal target is simply unaffected, but other effects of the spell (like "Draw 1") still resolve.

Sequence:

• Void Seeker is played targeting a unit at battlefield
• Opponent responds by moving the unit to base with Flash
• When Void Seeker resolves, the target is now illegal (not at battlefield)
• The target takes no damage
• The caster still draws 1 card

Nuances:

• This differs from spells like Falling Star and Icathian Rain, which create reflexive triggers that don't specify "at a battlefield", so they can still affect units that move zones
• Reflexive trigger spells (like Falling Star/Icathian Rain) only choose targets when the spell resolves and triggers are created, not when initially played