When playing a spell like Voidseeker that targets a unit, when do you declare the target - immediately when playing it or when the chain resolves?

Ruling: Targeting happens when a spell or ability is put on the chain, not when it resolves.

Nuances:

• Spells with "do this" wording (like Icathian Rain) work differently - the spell itself doesn't target when cast, but when it resolves it creates multiple instances on the chain that each require targets when placed