Shen, Eye of Twilight

Keywords in this card's text

Action

Also lets you play the card or activate the ability in an open Showdown, including on your opponent's turn. Its normal timing still works.

Action alone does not let you respond to a spell or ability waiting on the chain.

Tank

Assign lethal combat damage to that side's Tanks before its non-Tanks. The player assigning damage chooses the order among multiple Tanks. This does not protect other units from targeted spells.

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