I'll post my draft here instead of the comments so it'll be easier to discuss if there are any problems, if thats okay with you of course.
By the age of Dark Souls 3 Gwyndolin has become the main figure of authority and worship in Anor Londo, which by this time is now known as Irithyll of the Boreal Valley, taking his fathers title as King of the Gods. As well as continuing to watch over those who link the Fire, Gwyndolin appeared to be tracking down descendants of the gods who fled Anor Londo millenia prior and bringing them back Irithyll. The exact reason as to why he was doing this is unknown, though it could be that he was trying to ensure that the power of the gods continued to live on.
He ruled alongside his apparent sister; Yorshka, a young crossbreed girl. Although the exact nature of their familial ties is unclear, it is known that Gwyndolin thought very highly of her as he provided her with his own personal Chime as well as her name.
A former denizen of the Painted World and ally to Saint Aldrich of the Deep; Sulyvahn, would later inflitrate the city of Irithyll and the Darkmoon system in order to usurp it from within. It is highly implied that Sulyvahn worked his way up the power system in Irithyll, all the while turning the Knights to the worship of the Deep.
Gwyndolin would later become stricken with an illness so severe that he could no longer lead his own Covenant or the city. While it is unknown exactly why he fell sick, it is possible Sulyvahn was invovled. As he was too sick to lead his own Covenant, it's leadership passed to Yorshka, who was shortly thereafter imprisoned by Sulyvahn who took complete control of the city and claimed the title of Pontiff. The God of the Darkmoon was then imprisoned himself, inside the the ruins of the old cathedral of Anor Londo.
After Aldrich was revived as a Lord of Cinder, he quickly abandoned his throne and journeyed to Irithyll in order to consume the imprisoned god. Once he arrived, he slowly began to slowly consume Gwyndolin, who was most likely powerless to fight back, both physically and spiritually, gainging access to his knowledge of Priscilla and Nito while he was dreaming. 'However, the Unkindled arrived halfway through consumption and Aldrich took control of Gwyndolins body/power in order to defend himself as he was vulnerable while consuming the god. Gwyndolin was released from his gruesome fate when the Unkindled slew the Lord of Cinder, ending the gods torment and finally allowing him to die.
I'll list my sources for you:
(First paragraph source: Yorshka's dialogue, Irithyllian character precept lore, "King of the Gods" is Gwyndolin's title in the Japanese version of the game, Irithyll is a Darkmoon city as evidenced by the images of Gwyndolins iconic Crown found in the stained glass windows of the cathedral as well the many statues of Gwyndolin throughout the area and Greirats dialogue.)
(Second paragraph source: Yorshka's Chime)
(Third paragraph sources: Soul of Pontiff Sulyvahn, Snap Freeze, Pontiff Knight Curved Sword, Cathedral of the Deep Evangelist in the Irithyll courtyard.)
(Fourth paragraph sources: Soul of Pontiff Sulyvahn, Yorshka's dialogue)
(Fifth paragraph sources: Cinders of a Lord Aldrich, Soul of the Deacons of the Deep, Darkmoon Bow, Lifehunt Scythe. The clear evidence for it being Gwyndolin's original body and not simply a new one created by Aldrich, is the sound files for the boss, the general state of the Gwyndolin body model which is decaying in many places and the cut "Gwyndolins Finger" item, which you were supposed to originally get after the fight which you could give to Yorshka who would confirm that the finger was indeed Gwyndolin's.)
The sound files depict someone in pain both during the fight and when the boss is in a non-hostile state. Aldrich would not be experiencing pain or making these noises if the body was simply one he created.
I must apologize for not making myself clear; when I asked for a draft, I only meant for the line that said Aldrich taking the appearance of Gwyndolin. In any case, I read your draft and to me it looks well sourced and well written, so I believe it could be added to the "Lore" section of Gwyndolin's page and replace the second paragraph.
Unfortunately, I'm currently occupied in some real life affairs and/or focusing on other projects to take onto my own the decision of adding your draft to the page. I recommend you to visit our official Discord server and show it to our other staff there, on the #wiki_editing channel. Just make sure to explain where it should go and advice you will be posting a big amount of text first; then ask them if they can proof-read it, and if accepted, just remind them to add all sources/references you provided.
Thanks for your contributions and please let me know how it went.