Oryx, the Taken King was an incredibly ancient Hive who served as their god-king for billions of years. He alone possessed the ability to Take, converting sentient creatures into thralls known as Taken. He claimed this power for himself after casting down his god Akka for defying it's own law.
Two years ago Oryx came to this system aboard his Dreadnaught to avenge the death of his son Crota at the hands of a Guardian. Thus began the Taken War. Eventually that same Guardian struck down Oryx's physical body, banishing him to the Ascendant Realm. Over the course of a long-term operation, Guardians defeated the Court of Oryx, broke into the Taken King's throne world, and felled the monster for good.