Yun-Yuuzhan, also known as the Cosmic Lord, functioned as the primary deity and Creator God for the Yuuzhan Vong.
The legends say that Yun-Yuuzhan gave up pieces of his own physical form in order to create the universe, the minor gods, and the Yuuzhan Vong themselves. To remember these sacrifices, the Yuuzhan Vong would sacrifice their own body parts, as well as living creatures, as a way to honor his original act of creation. He was formally honored on the most sacred days of the Yuuzhan Vong calendar, with these dates being reserved for rites and sacrifices dedicated to him.
In addition, the Yuuzhan Vong religion held that the creator god possessed all of the universe's knowledge but kept it hidden. However, Yun-Harla stole some of this knowledge from him, and then passed it on to Yun-Ne'Shel, who in turn gave it to the Yuuzhan Vong. Consequently, the shaper caste had the belief that knowledge could not be created, and innovation was seen as a heretical act.
The Yuuzhan Vong took their name from him, and it was considered disrespectful to call them simply "Vong" (as was often done by inhabitants of the galaxy). Only the Supreme Overlord of the Yuuzhan Vong had the authority to speak to him, and the Supreme Overlord underwent ritualistic scarring and modifications to resemble Yun-Yuuzhan.
Yun-Yuuzhan's character was based upon Yuuzhan'tar, which served as the sentient homeworld for the Yuuzhan Vong. The Order of Yun-Yuuzhan, a religious organization, was named in honor of the Cosmic Lord.