When Leona attacks and triggers her stun ability, can an opponent react before the Leona legend ability resolves to put a buff counter on her?

Ruling: Yes, opponents can react between Leona's "When I attack" ability resolving and her legend ability resolving. If they have a reaction available, they can potentially kill Leona before she receives the buff counter.

Sequence:

• Leona's "When I attack" ability goes on the chain and can be reacted to
• Once it resolves, the Leona legend ability goes on the chain
• Opponents can react to the legend ability before it resolves
• Priority is passed between both players before a chain item can resolve