Carl's mental-defense skill, trained to level 16 mid-battle, granting immunity to mind magic and sight of enemy mages.
Under a coordinated psionic barrage from The Madness's mind mages, Carl's Mind Balance was declared overwhelmed as ten more instances of Endless Thread of Distraction plus Self Harm and Intrusive Thoughts piled on. Standing rooted in an All Tree and coached by a familiar dwarf's voice, he pushed the skill to level 16 — earning the achievement You Now Tread in the Realm of Gods for training it past 15 using battlefield conditions rather than potions.
At 16 the skill simply woke him and made him functionally immune: dozens of mind spells crashed against him without purchase, he could now see the enemy mind mages scattered through the opposing army and paint them for missile fire, and it outright negated Eris's floor-wide Bear Witness while everyone around him froze.
The skill that turns Carl's mind from the enemy's chosen target into the reason he is the last one standing.