A defensive spell Carl casts on himself for protection during the train-cab brawl with the Daughters.
Wisp Armor is a defensive spell in Carl's kit, which he casts on himself as Hekla's betrayal erupts into a melee in the Vermillion train cab — bracing against Eva's sabers and the magic missiles and electrical bolts of two Daughter mages who rush in. It's one of the survival layers he throws up (alongside his gauntlet and damage-reflect) while trying to keep Princess Donut and the unconscious Katia Grim alive.
A protective magical armor effect wreathing the caster.
A defensive buff Carl uses to survive the ambush in the train cab.
Also appears in books you've already read — spoiler-safe to their endings: