Burrowing Worm Mandible

Source
This item is a potential reward from the following Class Hall missions:


 * Critical Crops
 * Evergreen
 * Hazardous Materials
 * Feltotem Hounds
 * The Pale Predator
 * Hide as White as Snow
 * The Mountain Stirs

It is also a quest reward from normal Broken Isles world quests and can be found inside Curious Wyrmtongue Cache.