The Guardian once stood vigil over Mount Arreat. Death was not the end of his vigil, and he chose to bind his spirit to the mountain in order to protect it. When the mountain was destroyed, he "died again," and his soul wound up in the Pandemonium Fortress, alongside the other spirits.
Twenty years later, the Reapers attacked Sanctuary. Ordan instructed the Guardian and other champions imprisoned within the fortress on how to advise their allies. A Barbarian nephalem encountered the spirit of the Guardian while trying to defeat Malthael. They felt a kinship with the Guardian, wondering if they were an ancestor of his/her's. The Guardian stated that they spoke for the Barbarian's ancestors. The Guardian told him that the spirits were imprisoned in the fortress, and bid the Barbarian free them. They would give their ally the power of death, but defeating Malthael would be the Barbarian's task alone. When asked what his real name was, and if he were one of the Ancients, the Guardian stated that the Barbarian before him would know his name well, but it was a memory that meant nothing to him now.
The Barbarian became one with death, and the Guardian was pleased that the Children of Bul-Kathos would have a new champion to look up to. He was ready to pass beyond, to see his family again.