Imogen's shape-warping spell — a village-killer that leaves its survivors permanently changed and oddly immune.
Transfiguration is Imogen's signature spell, and its most visible product is walking beside Carl on the trail. She cast it on the were-castors' village, killing almost everyone; those who lived through it became the Were-Castors, and they have carried the shape ever since.
The spell turns up again years later as one of the identifiable layers in the cloud of Scolopendra's Nine-Tier Attack, where it killed or transformed most of the animals in the city across the river. Holger's theory for why he and his people shrugged that cloud off is that the same spell cannot take a second time — they had already been transfigured once. He also passes on a rumor that Imogen tried the spell on the giant centipede herself and it simply did not work, which would explain how a spell of hers ended up in Scolopendra's stored, reflected barrage. Holger is careful to note he cannot be certain, because there was plenty else in that cloud: spores, a rage spell, all sorts of nasty stuff.
The spell that created the were-castors, one of the nine tiers of Scolopendra's attack, and the load-bearing detail of the region's disaster history.