Mithrax has determined that the energy of the City is being siphoned by Quria for an unknown reason . He says he has been pursuing Quria's signal through the domain, but its exact location keeps slipping through his fingers. When he gets close, the Vex rise to break his focus.
"It may seek the destruction of your City, or of your Vanguard," he says, "or something even greater may be at stake."
He asks you to strike at the Vex and disrupt their network so he can find Quria through the chaos.