Darius, Hand of Noxus

Keywords in this card's text

Reaction

Includes Action's timing and also lets you respond to a spell or ability before it resolves.

A Reaction can start a chain; it does not have to answer another card. It cannot interrupt an effect that is already resolving.

Legion

The linked Legion text applies if you have played another card this turn. It is not limited to your second card; activating an ability or channeling a Rune does not count as playing another card.

How Focus and Priority decide when Action and Reaction can be used