Destroy 6 Highborne Apparitions and 6 Highborne Lichlings before returning to Loh'atu in Azshara.
Long ago, the Highborne were some of the most powerful students of the arcane the night elves ever produced. The corruption that comes along with such studies caused much harm to night elf society and greatly affected all of Azeroth... as I'm sure you well know.
I promised the Sentinels that I would release the spirits from their mortal bonds as I find them. If you would do the same, then I am sure I can reward you some.
The spirits haunt the Shadowsong Shrine to the northeast of here.