Dark Sun Gwyndolin

Dark Sun Gwyndolin was the youngest son of Lord Gwyn, prince of Anor Londo and an integral character in the lore of Dark Souls.

Personality
Gwendolin seems to hold a "deep adoration for the sun", including his father and livelihood. He has a deep desire for his father's affections, and not only does he cast in illusionary sunset over Anor Londo to create the illusion of a prosperous city, but he even guard's his father's tomb himself, attacking the Chosen Undead should they trespass, even though he is fully aware that the tomb is empty.

Given his power of illusions, Gwendolin is appropriately a masterful schemer. His illusion of Gwenevere is used to trick the Chosen Undead into linking the fire and continuing his reign in Anor Londo.

History
Eons after the war between the gods and the dragons, Gwyn established his own kingdom in Anor Londo where he married and had children. Gwyndolin was the youngest of his sons, and because he possessed inherent abilities associated with the moon (which was considered a feminine trait), was raised as a daughter.

When Gwyn left to the kiln of the first flame to link the fire and stave off the Age of Dark, he told his family to look after the humans in his absense. Not wanting to let him down, Gwyndolin sought to keep the humans safe and soon had humanity coralled to the north or exiling them to stone prisons to keep the curse of the undead under control.

All other deities in Anor Londo would soon leave as well, horrified by what was happening. Gwyndolin however decided to stay in the now abandoned city of the gods. Using his power over illusions, he creates an illusion of his sister Gwenevere to help guard over the city.

Despite his lackluster status as a deity, he does manage to gather a group of followers called the Blades of the Darkmoon, a covenant created to find and hunt down those who are guilty of crimes against the gods and claiming vengeance. They accomplish this by reading out a list from the Book of the Guilty from Goddess of Sin Velka.

Dark Souls
When the Chosen Undead reaches Anor Londo, battling through guards, demons and royal knights Ornstein and Smough, the Chosen Undead reaches the illusion of Gwenevere, who will tell the player that they are tasked at linking the first flame and staving off the Age of Dark. Should the player attack the illusion, Gwenevere will disappear, undoing the illusions cast over Anor Londo.

For this blasphemy against his family, Gwendolin angrily reveals his presence and will proceed to send his covenant to kill the Chosen Undead. He then fights the Chosen Undead himself when trespassing into Gwen's tomb, where Gwendolin supposedly dies.

Dark Souls III
It is revealed that Gwendolin survived the events of Dark Souls and is living out his days in what was left of Anor Londo.

After Aldrich was revived as a Lord of Cinder, he began dreaming of the Old Gods and sought to devour them. Sulyvahn traveled with him to Anor Londo and managed to capture Gwyndolin, who sacrificed himself to save Captain Yorshka. She managed to evade capture but Aldrich devoured and assimilated Gwyndolin, who became the upper body of the abomination, as well as Gwyndolin's abilities.

After devouring Gwyndolin, he fell into a deep slumber and dreamed about a young, pale girl in hiding.

Trivia

 * It is implied through the illusion of his sister that Gwendolin is in-league with Kingseeker Frampt, one of the primordial serpents.
 * Dark Sun Gwendolin is actually the head of two covenants at once. The first is the Princess Guard, a covenant ran by his illusion of his sister, and the Blades of the Darkmoon in his name.
 * It is implied that Gwyndolin was shunned by the family, or at the very least was treated as an outcast. Unlike his parents and older siblings, there are no signs of statues dedicated to Gwyndolin, the only traces of loyalty among mortals being from the Blade of Darkmoon covenant, a group founded by Gwendolin himself.
 * It is never explained why his lower half it composed of sepernts supporting him, but many have theorized that he was experimented on by Seath the Scaleless, as not only does Seath and his Pisacas have similar features, but his boss music is the same as the Moonlight Butterfly, a monster created by Seath. Other believe that this was simply a means of making himself look more intimidating.
 * It is very likely that the Gwendolin the player faces and defeats in Dark Souls was an illusion, as it is revealed that he survived long enough to fall victim to Aldrich by the events of Dark Souls III.
 * Given his title "Dark Sun", Gwyndolin shares much in common with many other outcast gods of mythology.
 * Despite being the eldest, Hades of Greek Mythology was the only member of his family not considered an Olympian, nor was he considered an idol of worship.