In Dark Souls, Gwyndolin -- also known as the Darkmoon deity and the Dark Sun -- is Gwyn's youngest child and the only god who still remains in Anor Londo. In the city's palace, he has created an illusion of his sister Gwynevere.
Gwyndolin is the leader of the Blade of the Darkmoon covenant, and although he is a male, due to his magic and association with the moon he was raised as a daughter.
There are two ways to fight Gwyndolin. One is attacking the illusion of Gwynevere. The other is wearing the Darkmoon Seance Ring in the room that houses Anor Londo's second bonfire. This will reveal a secret path, and if one ventures into it one can choose either to fight the Dark Sun or to enter his covenant, Blades of the Darkmoon. After defeating him he drops:40,000 souls, Soul of Gwyndolin, and unlocks the Moonlight Armor Set.