Basically we (our character, the Godslayer) used Vex technology that Osiris built to time travel and save him before he lost his light and died. If you wanna know how Saint ended up dying in the first place, the Vex created a Vex Mind designed specifically to drain Saint's Light. Saint destroyed this Mind but lost his Light in the process. He made his last stand and killed a f*ck ton of Vex before eventually being overrun and killed.