I own an-windr. I just maxed him and put a talent point on him to use in future gaurdians challenges. I have been experimenting with him to see how his special skill works. As it turns out he never does any damage to the enemy. He skill duration ends and… nothing.
Later I realized that his damage happens at the end of the enemy’s turn when the buff expires.
I just tried with unleveled An-Windr in S1 17.1 and it did damage. Seems like at least mine is doing its job. But i must admit that the symbol for the damage dealt is somewhat confusing: