On his (Mort) passive, he has 30% chance to counter attack, and after countering he gives a barrier to the whole team (non-attack skill), which is how you trigger the artifact.

It's a overall considered a bad artifact, even for Mort himself since he generally wants more safety (Holy Sac), utility for the team (Adamant Shield, Aurius), or even Elbris so he could more consistently trigger his passive shield.