Mithrax speaks to you through the Splicer Servitor that his House of Light liberated from Eramis's House of Salvation.
He explains that he is a Sacred Splicer, one specially trained to interface with the Light threaded through the Vex network. He is so impressed with your mastery of the Light that he gives you a custom-made Splicer Gauntlet to help you focus your own abilities. He is willing to guide you in your journey to become a Sacred Splicer, while Ikora, Osiris, Lakshmi-2, and Saint-14 assist when they are able.
He asks that you bring him Vex data so he can begin to decode the mysterious Paradrome Cube you claimed from the Vex on Europa. He believes it will reveal how to lift the oppressive simulation that hangs over the City.
"Fight bravely," he says. "Free your people from this Endless Night."