The Army of the Light is a brand new faction that is found in Argus in patch 7.3 of World of Warcraft, and acts at the main World Quest faction in the area.
The main way to gain reputation with the Army of the Light is by doing World Quests in Argus, and expect them to take roughly as long as the other Legion factions to hit exalted. You also get some normal one-time quests that will give you a certain amount of rep with them while questing in Argus, so you don't have to begin from 0/3,000 Friendly. And just like the other factions, the Army of the Light has Emissary Quests as well, so make sure to do these for a nice rep boost.
At Exalted you can purchase the Lightforged Warframe mount from their faction vendor, but it costs a total of 500,000 gold! However, there's also a few mounts that do not cost any gold, and the Gleaming Footlockers you get from the Paragon reputation rewards from the faction can actually give you the follow 3 mounts:
- Avenging Felcrusher.
- Blessed Felcrusher.
- Glorious Felcrusher.
These all use the fel Elekk model, and look really cool!