Carl's crowd-breaker, cast once the weasels are visible enough to scatter.
Carl casts Fear partway through the Night Weasels ambush, after his Smoke Curtain is down and once damage and Mongo's Earthquake attack have made the pack visible enough to target. He follows it immediately with a Banger Sphere.
It is a small moment in a fight the Were-Castors largely win on their own — Clint Smashgrab and Holger do the lion's share of the killing — but it is part of the pattern of Carl fighting a pack he cannot see by making the battlefield unpleasant rather than by aiming.
Carl's area crowd-control spell in the trailside ambush.
Also appears in books you've already read — spoiler-safe to their endings: