An extremely rare herb, one is needed for each flask that an alchemist crafts. Only one black lotus is up at a time, and it can be found in Silithus (one north of AQ gates, one in crystal vale) and Winterspring (one at Mazthoril blue dragon area, and one south of the main road in the western Furblog area). Apparently also one in Eastern Plaguelands, I never saw it.
Unlocking the Lightforged will require you to complete the aforementioned Argus storylines and earn the “You Are Now Prepared” achievement. In addition, you’ll have to earn Exalted status with the Army of Light. As with the Argussian Reach, this one could take a couple of weeks, as it is time-gated. However, the main story quests should help you at least reach Friendly status with the Army of Light.