Scolopendra's transformation event that turned an entire chee kindergarten class into were-castor beavers.
According to the story Clint tells, Scolopendra's nine-tier attack struck the dungeon's sixth floor and turned his entire kindergarten class of Chee into a group of shifters called Were-Castors, which are really just beavers. The transformed children then spent the next few hundred years growing up, becoming warriors, and fighting the High Elf and the Naiad Confederacy.
The attack's effects were not permanent for everyone: because Edgar's tattoos on Signet reverted their bearers to their pre-attack forms, Clint and Miss Nadine returned as chee when they became Flesh Golems, undoing centuries of transformation and growth.
Historical catastrophe that created the were-castors and shaped the sixth floor's wars.
Also appears in books you've already read — spoiler-safe to their endings: