After you unlock Labyrinths in Patch 12.1.5 and you complete the introduction quest, a new NPC, Wah'du, will be at the entrance and have a quest for you. This quest will task you with going to a chamber in the Labyrinth and finishing the objective there on Tier 8 or above.
Completing the quest will earn you one Mythical Soul Fragment and 2,000 Undercoin. On PTR, Wah'du will then have another similar quest for you, but for another zone, but it's likely that on live servers these quests are Weekly Gated. Once you complete 3 quests and have 3 Mythical Soul Fragments, you can combine them to get a Myth 1/6 piece of gear. The best part is that you can choose the slot that you want it for!
If you're looking for a piece of Myth track gear, this is a great way to spend some Myth crests once Patch 12.1.5 comes out.
