Translates roughly as The Celestial Venerable of the Primordial Beginning. He is also known as Tian-bao-jun (Lord of the Heavenly Jewel). He is one of the Taoist San-qing ('pure ones'). There is San-qing one for each of the three Taoist Heavens, and Yuan-shi tian-zong belongs to Yu-qing (the Heaven of Jade Purity).
He came into being at the same time the universe did, as a result of the merging of 'pure breaths', and he is the one responsible for creating Heaven and Earth. He originally ruled over both, but he eventually gave them over to Yu-huang, the Jade Emperor. At the start of each aeon he instructs his subordinate deities on the Ling-pao ching (the Scriptures of the Magic Jewel). He also occupies himself with the rescue of souls from the various hells.
Yuan-shi tian-zong is said to be eternal, limitless, and invisible.