If an opponent plays Hextech Ray targeting a card on field, can they change the target if the defending player responds by playing a unit from hidden (like Teemo in trap) before Hextech Ray resolves?

Ruling: No, they cannot change targets. The target is chosen and locked in when Hextech Ray is finalized to the chain, before the opponent has a chance to respond.

Sequence:

• Player plays Hextech Ray and chooses target when finalizing it to the chain
• Target is now locked in
• Opponent can now respond (e.g., by playing a unit from hidden)
• Hextech Ray resolves with original target

Nuances:

• The unit being played from hidden must not have been hidden on the same turn (summoning sickness applies)