Drogön Chögyal Phagpa
Drogön Chögyal Phagpa (1235-1280) was one of the Five Founders of the Sakya Tradition of Tibetan Buddhism. The last part of his name in Wylie transliteration of Tibetan is 'Phags pa, and one will often see him referred to as Phags-pa.
Chögyal Phagpa's link to conlanging comes from his position as tutor to Kublai Khan and his Empress. The Khan wanted a script that could be used to unify the myriad languages in his empire, and Chögyal Phagpa created a new writing system based on Tibetan. According to the lama's biography, as a token of appreciation for this creation, the Khan gave Phagpa the right to rule over three Tibetan provinces.
Unfortunately, the new script met with resistance and was never used extensively (similar to many attempts of this kind). Most official documents continued to be written in Chinese characters. Omniglot has a nice page outlining Chögyal Phagpa's script.
Note: The image of Drogön Chögyal Phagpa at the right is a detail from an image at Himalayan Art. The original is from the Collection of Shelley & Donald Rubin and is used here under the Fair Use provisions of the Copyright Act as outlined here.