In the 12.0.5 patch, we’re introducing the new achievement Midnight Keystone Myth - Season One. This new achievement rewards a saddle that can be exchanged for a mount of your choice once you’ve attained a Mythic+ Rating of at least 3400 during Midnight Season 1.
About an hour ago, we enabled the Midnight Keystone Myth Achievement, and it has been going out to players who qualified.