Ansillon

Ansillon was the name of a greatly respected Time Lord, who was an old friend to the Doctor.

The Fourth Doctor remembered first meeting him when Ansillon was a hermit on a mountain. For a time, Ansillon was the solitary guardian of the Caves of Perdition.

In other lives, he served many different roles: as Ku'an Impe, he was the Watchman of the Dark, and as Uba Danth, he was the High Priest of the Riven Temple of the Spliced, "among many other names" across his incarnations.

Later in life, he reclaimed the name Ansillon and became the Provost of the Recusary. Ansillon remained associated with the High Council and the Keeper of the Matrix during his time at the Recusary, and gave his testaments in a series of codeces. (AUDIO: Blood of the Time Lords)

Behind the scenes
Since the Doctor says he first knew Ansillon as a hermit on a mountain, and Ansillon notes that he's familiar with the circumstances of the Third Doctor's regeneration, Blood of the Time Lords strongly implies that he is K'anpo Rimpoche and/or the Stranger on Mount Perdition.