How does targeting work with Hidden Blade when the opponent responds by moving the targeted unit?

Ruling: Hidden Blade targets a unit at a battlefield. If the opponent responds by moving that unit (e.g., with Flash), Hidden Blade fails target legality on resolution and does nothing - no unit is killed and no cards are drawn.

Sequence:

• Hidden Blade is played and a target is chosen (a unit at a battlefield)
• Target is locked in the chain and cannot be redirected
• Target legality is checked again on resolution
• If the unit has moved to an illegal location (like base), the spell fails to resolve and no effect occurs

Nuances:

• You cannot redirect Hidden Blade to a different unit once the target is chosen
• If the target becomes illegal, no controller can be determined, so no one draws cards
• If the unit moved to a different battlefield (not base) and Hidden Blade was played from hand (not hidden), it would still function
• Cull of the Weak does not target, so you must sacrifice a unit on resolution regardless of what units existed when it was put on the chain