Scale the Vex Citadel and hunt down the source of Venus's blight. Complete mission "Entropy's Pinnacle."
"The blight of Oryx has touched the heart of our worlds. From the depths of the Cosmodrome to the apex of the Vex Citadel, Taken claw at the Light. Seek out the source at the top of the Citadel, and end the Darkness that dwells there."