One of the first quest steps in the Evoker Legendary quest chain is to acquire 3 items from various crafters! Here are the datamined reagents for the crafts.
Once an Evoker is on the quest, every 30 minutes they can teach a crafter a recipe to craft a part of the pattern for 30 minutes. Each of these mats requires the Evoker to do various pieces of Patch 10.1 content.
Temporal Vestigial may drop from open world content, as a World Boss dropped 10 and rares drop 3.