If I move a 3 might unit into a battlefield and the opponent plays Hextech Ray, can I play Arcane Shift targeting one of their units at a different battlefield to save my unit by moving it to base?

Ruling: No, you cannot play Arcane Shift to save your unit from Hextech Ray. Both spells are Action speed, and Action spells can only be played in an OPEN state (when no chain exists). Once Hextech Ray is played and placed on the chain, the state becomes CLOSED, preventing you from playing Arcane Shift in response.

Sequence:

• Opponent plays Hextech Ray targeting your unit
• The chain is created and state becomes CLOSED
• You cannot play Arcane Shift (Action speed) because only Reaction speed cards can be played while a chain is active
• Hextech Ray resolves and deals damage to your unit
• If your unit dies, it goes to trash and cannot be targeted by Arcane Shift even if played later

Nuances:

• Even if your only unit dies during a showdown, the showdown continues and you can still play Action spells afterward (when state is OPEN again) to target opponent's units
• Arcane Shift can target a friendly unit and an enemy unit at different battlefields (they don't need to be at the same location)
• You would need a Reaction speed card to respond to Hextech Ray on the chain