If Leona is removed from a battlefield before her stun trigger resolves, does the trigger still stun an enemy unit at that location?

Ruling: If a unit is removed from a location before their trigger resolves, the "here" it refers to might no longer have valid targets. If the unit has been defeated, the ability will resolve to no effect.

Nuances:

• The opponent cannot play Challenge during the Initial Chain (when attacking/defending triggers occur) because Challenge is an Action speed card
• Attack triggers resolve before Action speed cards can be played