R'hllor, The Lord of Light

Traitor to the Fourteen, R'hllor is not a true god in his own right, but an aspect of Urrax originally designed to gather the worship of slaves in Valyrian mines as well as to serve as a weapon against the eldritch.

With the Doom of Valyria, the Fourteen were devastated, but the faith of R'hllor thrived. Through R'hllor, the Fourteen were able to siphon off enough power to survive, but the balance between them was too skewed, and eventually R'hllor desired to become a god of his own and usurp his masters.

Known Priests/Priestesses
High Priestess Kinvara was the former head of the entire religion of the Lord of Light. As such, the aspiring god invested her with a great many blessings, making her smarter than her fellow man, swifter of tongue and clever of thought, with a beauty that bewitched the mind. Her greatest task was to attempt to convert the Dragonlords of Volantis away from the Fourteen, specifically Emperor Aegon Targaryen as he was the most fitting candidate for Azor Ahai. After the Dothraki Invasion, she arranged weekly meetings with Aegon and Daenerys to "discuss theology" wherein both sides attempted to convert the other. She eventually resorted to seduction, seeing as merely talking hadn't worked. However, she failed. Kinvara ended up being the one seduced by Aegon, and in the middle of sex he managed to get her to forswear R'hllor and proclaim the Fourteen as the true gods.