When casting a spell that targets units, do you declare targets immediately when casting or when resolving? What happens if the target becomes invalid? Also, can you play Hidden cards in response to Deathknell triggers to save them from being trashed?

Ruling: You must declare targets immediately when casting a spell, and those targets are locked in - you cannot retarget at resolution. If a target becomes invalid (e.g., moved by Flash), the spell is not canceled, but parts referencing that target do nothing. For spells with multiple targets, valid targets still resolve normally. You can play Hidden cards (including Zhonya's Hourglass) in response to Deathknell triggers after Combat Cleanup, which saves the Hidden card from being trashed.

Sequence:

• Targets are declared when the spell is cast
• Targets are locked and cannot be changed at resolution
• If target becomes invalid, spell still resolves but effects requiring that target do nothing
• Combat Cleanup occurs (units die)
• Deathknell triggers resolve (after cleanup, before step 444.2)
• Hidden cards can be played during Deathknell resolution
• If Hidden units are played after attackers would recall, a new combat starts with remaining units

Nuances:

• Spells like Void Seeker will still draw a card even if the damage target is gone
• Multi-target spells like Singularity still affect remaining valid targets
• Falling Star and Icathian Rain use reflexive triggers and work differently
• Playing Hidden units during Deathknell starts a new combat after the recall step would have occurred
• The new combat doesn't change control - defender remains defender, and opponent's abilities like Assault reactivate
• Playing Hidden units prevents conquest but doesn't recall attackers retroactively