The force at the center of the Vex network has a name: Quria.

Mithrax pledges to use his abilities as a Sacred Splicer to search through the Vex domain until he finds its lair. He tells you he imagines his Light as a beacon, something to use to guide others through the darkness of the world.

He believes you were given your Light to use as a weapon… and Quria is the target.