A defensive spell Imani and Prepotente urge Carl to cast against the psionic barrage.
With Carl pinned under stacked psionic infections, Imani relays Prepotente's instruction twice: Prepotente is almost there and can shield Carl, but Carl needs to cast Wisp Armor himself in the meantime.
Carl never casts it. By the time he is on his feet, his Mind Balance has reached level 16 and the incoming mind spells are crashing against him without finding purchase, so he tells Imani there is no need as the Emberus Ring on his finger cools.
The emergency defense Carl is told to raise, made unnecessary by his Mind Balance breakthrough.
Also appears in books you've already read — spoiler-safe to their endings: