Shimmering Timewarped Crystal

Shimmering Timewarped Crystal starts the quest.

Source
This item drops from the following bosses on Timewalking mode:
 * Teron'gor in
 * Gug'rokk in Bloodmaul Slag Mines
 * Yalnu in the Everbloom
 * Skulloc in Iron Docks
 * Ner'zhul in Shadowmoon Burial Grounds
 * High Sage Viryx in Skyreach