Diana, No Longer Human

Keywords in this card's text

Ambush

Play the unit with Reaction timing at a battlefield where you control a unit—even if you do not control that battlefield. Normal costs and exhausted entry still apply unless card text changes them.

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.

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