Legion Keymaster

Achievement should have been made a feat of strength as Legion mythics no longer drop keystones and thus this achievement is no longer possible to get. https://us.battle.net/support/en/article/73499

This achievement is earned after you are done with all keystoned mythic dungeons in the Broken Isles.
You don't need to finish the run on time, you can take 2 hours and it will still count towards this achievement. Of course, you are still going to end up with a depleted keystone.
Violet Hold is not a requisite because, by its nature, it's already sort of a "timed event" even in non-keystone versions.

I find it quite unfair that they removed the possibility to get it without further notice. I was missing one and if I had known, that this achievement would be removed (like The Chosen, Ashran, SotA and others), I would have most definitely tried to get it before launch.