When does Leona's stun effect require declaring a target - at initialization or resolution?

Ruling: Leona's stun effect targets, and targets must be declared when you put the ability onto the chain (at initialization), not at resolution.

Sequence:

• Leona's effect goes on the chain when showdown starts
• You must immediately declare which enemy unit at the battlefield is the target of the stun
• The stun resolves later when the ability resolves from the chain

Nuances:

• An ability targets if it must choose a specific game object to affect (not if it affects all objects of a set)
• Even if there is only one valid target, you still must declare it as the target