One of the new mounts added in the first PTR of Patch 9.2 is the Mythic+ Season 3 mount: the Wastewarped Deathwalker, a red recolor of the other Deathwalker mounts Sintouched Deathwalker and Soultwisted Deathwalker.
It is raw swirling energy bound together into a cohesive form, bent to the will of those strong enough and determined enough to bind it.Currently the Keystone Master Season 3 achievement hasn't been datamined, but the requirement is likely the same of the Season 2 achievement:
Read more on wowhead.com