The Priestess' Moonsaber mount is an Alliance-only mount added in patch 8.1. It uses the model of the updated nightsabers, but it looks really unique compared to most of them, so it's a very cool mount to have.
The only way to get the Priestess' Moonsaber mount is to buy it from Provisioner Stoutforge in Boralus next to the Warfront portals. She sell this mount for 750 x 7th Legion Service Medals.
You need to get all 750 7th Legion Service Medals on a single character, since you cannot send these in the mail, so be prepared for a lot of farming if you want to get this awesome moonsaber.
There are currently four different ways to obtain 7th Legion Service Medals:
- Complete World Quests in Warfront zones when the Alliance controls it.
- Open 7th Legion Paragon Caches. You get one each time you get 10,000 reputation after having reached Exalted.
- Participate in Warfronts. The first win of each cycle gives you 15 Service Medals.
- Complete Faction Assaults. Doing the main faction assault quest gives max level characters 5 x 7th Legion Service Medals.
As you can see, you can't directly grind these, but must instead do some chores every day to get the Service Medals you need. Most players will have to do daily things like WQs and Faction Assaults for a few months before they are able to afford to buy the Priestess' Moonsaber mount.