Infallible Mage is a light armor set. Your fully-charged Heavy Attack deals an additional 903 damage. Enemies you damage with fully-charged Heavy Attacks are afflicted with Minor Vulnerability for 10 seconds, increasing their damage taken by 8%.
This set applies the same effect as the Concussed status effect does. Concussed has a small chance to proc on Shock abilities or for example the Shock enchantment. With this set you can reliably keep up the Minor Vulnerability debuff on the enemy, guaranteeing the 8% damage increase. You need to do a fully charged Heavy Attack to get the effect to activate on the enemy.
The Infallible Mage set is part of the base game and can be found in the Craglorn Trials such as Aetherian Archive, Hel Ra Citadel and Sanctum Ophidia. Each set item is bound on pickup. You can acquire light armor, jewelry and weapons for the set in the Yokudan motif style.