The Crusades happened after the Serpents Betrayal

I'm sure this has been pointed out already but the God Devouring Serpent's betrayal had to have happened before Messmer was "secreted away."

Black Knight Andreas Ashes:

"Ashen remains in which spirits yet dwell. Use to summon the spirit of Andreas, Knight-Commander of the Black Knights. The Black Knights were the primary force of Messmer's army. Their first leader was Andreas, a man endowed with great strength and command over the powers of the Crucible, and whose spirit in these ashes dwells. Though he remained a devout follower of Messmer after his flight from the Erdtree, he would rebel after learning of his liege's serpentine nature. His righteous stand was rewarded with imprisonment in an underground tomb."

This implies that a schism occurred shortly prior to the Crusades. Not only that but this would have been in the midst of Godfrey's reign as the Duelist Helmet reads:

"Bronze helm decorated with innumerable snakes. Worn by gladiators who were driven from the colosseum. The wearer becomes a slightly easier target for foes.

The snake is viewed as a traitor to the Erdtree, and the audience delighted in seeing these bronze effigies beaten and battered."

Godfrey was the one who encouraged the colosseum fights, a practice that would continue on into the Shadowlands.

Dueling Shield:

"An armament once used in ritual combat performed to honor the Erdtree—a custom that had somehow remained within the realm of shadow."

It was still fresh in his mind. Because of this I wondered if the betrayal of the Serpent was its siding with Rykard.

To infer it's duties while working alongside the Erdtree (hence it's betrayal) we can take a closer look at the Duelist Set specifically the Gravekeepers Cloak. He must have been a master of Gravekeepers!

Biblically this actually fits! In Genesis after tricking/teaching Adam and Eve about the apple the Serpent is robbed of its wings and legs and forced to crawl upon the ground. Not only that but it's also cursed to "feed upon dust," which is often interpreted as "Death."

"To prevent Adam and Eve from eating the fruit of the Tree of Life and living forever, they are banished from the garden upon which God posts an angelic guard. The serpent is punished for its role in the Fall, being cursed by God to crawl on its belly and eat dust."

Thus the saying "Ashes to ashes and dust to dust."

I believe that as a Gravekeeper the serpent would have had a form knowledge not only of death itself but it's conversion into power: Hexes.

Staff of the Guilty:

"A heretical staff fashioned from a smoldering, withered sapling that turns the blood of sacrifices pierced by it into glintstone. Similar to hex magic.

Sorceries are scaled with faith rather than intelligence when wielding this staff which enhances Thorn sorceries in particular."

A staff that converts life directly into Glintstone through "sacrifice."

Serpent God's Curved Sword:

"Curved sword fashioned in the image of an ancient serpent deity and tool of a forgotten religion practiced on Mt. Gelmir.

Formerly used to offer up sacrifices, this sword restores HP upon slaying an enemy."

Thus did he impart to Rykard this same knowledge.

Magma Shot:

"One of the sorceries developed from the magma of Mt. Gelmir.

Fires a lump of magma that explodes on contact. Charging enhances potency.

After discovering the ancient hexes of Gelmir, Rykard, son of Queen Rennala, brought them back into practical use as new forms of sorcery."

Rykard REVIVED the ancient hexes of Mt. Gelmir. The same hexes practiced long ago in the Serpents sacrificial cult!

To sum it up the Serpent seems to have been deposed at one point for its betrayal. This betrayal being the consumption and conversion of life into power something that Marika would not at all have been ok with.

I'm still confused on a bit of the timeline prior to Messmer's banishment but I believe this to be a decent starting point. The serpent would have had to have been considered a traitor prior to the start of the Crusades and reviled in TLB likely throughout.

Please lmk your thoughts!