Blizzard has quietly removed the 
Ashes to Ashes meta-achievement, originally tied to the Legion expansion, in the latest Patch 11.1.7 PTR Build 60799.
The achievement, which appeared only a few weeks on the PTR, was intended to reward a new mount named Rites of the Felsoul. However, the mount was never added to the game files, and so we don’t have any model for it.
When the Legion meta-achievement first appeared on the 11.1.7 PTR, it wasn’t fully implemented. The achievement’s objectives included placeholder entries, many of which were tied to Battle for Azeroth content instead of Legion.
This strongly suggests Blizzard may have used the BfA meta-achievement as a template when creating Ashes to Ashes, and never finalized the Legion-specific requirements.
We’ll keep you updated on when this gets implemented again!