If I play Hidden Blade and my opponent retreats the target unit, leaving only my own unit at another battlefield as a valid target, do I have to choose my unit or does the card have no valid targets?

Question

Accepted Answer

Ruling: When you play Hidden Blade, you must select the target immediately. If the target is moved from the battlefield after being chosen (such as by Retreat), you cannot switch targets. Hidden Blade resolves to no effect, and the opponent does not draw a card.

Sequence:
- Play Hidden Blade
- Select the target (opponent cannot react before this)
- Opponent can react with Retreat to move the target
- You cannot change your target after the reaction
- Hidden Blade resolves to no effect

Nuances:
- Even though Hidden Blade resolves to no effect due to an invalid target, the opponent still does not draw a card because the target and its controller were invalid at resolution