Carl's opening move against an enemy he cannot see anyway.
Carl's first act when the invisible Night Weasels hit the trail from three sides is to drop a smoke curtain. It is a reflex more than a calculation — the weasels are invisible regardless, and the party's real eyes in the fight are the Were-Castors, who hunt by sound and smell and are unhindered by either smoke or invisibility.
Under its cover Carl snap-kicks weasels off the path, crushes one against a tree with his foot, casts Fear, and finishes with a Banger Sphere once Mongo's Earthquake attack has given him visible targets.
Carl's standard area-denial opener, deployed at the start of the weasel ambush.
Also appears in books you've already read — spoiler-safe to their endings: