Is it a part of The Darkness? It couldn't be Fallen, Hive, or cabal... But it could be the system or mind that controls the Vex. In one of the missions on Venus in campus-9 the ghost says that all the Vex are part of one huge system. You can't destroy it, it can destroy you, and it is 1 of four types of Darkness. So my answer is the system that controls the Vex.