Praedyth is stuck in a "cell" out of time. The Vex have trapped him. If you successfully rid the Vault of Taken, that is, clearing the enemies out of the Atheon room, Praedyth still lives. His Ghost had seen the Fallen's Golden Age, an Exo-Vex battle (possibly the Collapse), and the remnants of humanity fleeing across the stars. Just look up the Collective of Knowledge in the Search menu; you'll find it