If a card like Void Seeker tries to kill a unit I control, can I use a hidden Hidden Blade to kill my own unit instead? What happens to Void Seeker if there are no other legal targets, and does the player who played Void Seeker still pay the cost?

Ruling: Yes, you can respond by revealing Hidden Blade to kill your own unit before Void Seeker resolves. The player who played Void Seeker still pays all costs (including any deflect cost) when they first cast the spell. When Void Seeker resolves, it does as much as it can - the target is gone so it doesn't deal damage, but the caster still draws 1 card.

Sequence:

• Player A plays Void Seeker, pays all costs, and chooses the target
• Void Seeker goes on the chain
• You respond by revealing Hidden Blade targeting the same unit
• Hidden Blade resolves: you kill the unit and draw 2 cards
• Void Seeker resolves: target is gone so no damage is dealt, but the caster draws 1 card

Nuances:

• Costs are always paid when you first cast the spell, regardless of what happens during resolution
• This includes deflect costs if any were required