In
Oblivion and
Skyrim you can find a book called,
Manimarco, King of Worms. It speaks of the events that happened between him and Galerion such as thier time with the Psijics and Manimarco's necromancy and subsequent banishment. However, later in the book it speaks about a battle between Galerion and his mages with Manimarco (in which returned somehow, hmm) which took place at a mountain pass in an undisclosed location and time. The potential connection seen with this book and part 2 of the dlc is the notable mention of Lamp Knights in it. An excerpt from the book,
"Galerion beheld Mannimarco's rise through powers sublime,
To his mages and Lamp Knights, 'Before my last breath,
Face I must the tyranny of worms, and kill at last, undeath.
He led them north to cursed lands, to a mountain pass."
The location does say a mountain pass which may refer to the mountains that host the Writhing Wall as it leads East North East. Not seeing what's beyond the wall yet, it's most likley acursed and decaying from Worm Cults presence.
It mentions a familiar name, Lamp Knights; members of the Order of the Lamp, like those we've seen here in ESO. In theory based off occurances of Lamp Knights mentioned, ESO has had the most, including seeing them in game. Continuing in the book, it describes the battle between Galerion and Manimarco with mentioning The Worm has collected powerful magic artifacts Galerian demands to be surrendered, again not explained what for. This is another parallel we can see reflected in in part 1 of STWC as we collected a corrupted artifact of Meridia. The quote goes as follows,
"O those who survived the battle say its like was never seen.
Armored with magicka, armed with ensorcelled sword and axe,
Galerion cried, echoing, 'Worm King, surrender your artifacts,
And their power to me, and you shall live as befits the dead.'
A hollow laugh answered, 'You die first,' Mannimarco said."
Further in the book it mentions the supposed end which results in the death of Vanus Galerian and assumed defeat of the Worm King by, "The artifacts of death pulled from his putrid skeletal claw." This could mean he's resummoned as a lich we may fight at the apex of quest line. This is explained near the end of the book,
"His eyes burning dark fire, he opened his toothless maw,
Vomiting darkness with each exhalation of his breath,
All sucking in the fetid air felt the icy touch of death.
In the skies above the mountain, darkness overcame pale,
Then Mannimarco Worm King felt his dismal powers fail:
The artifacts of death pulled from his putrid skeletal claw."
In regards to the fate of Vanus Galerion in the following paragraph it says,
"A thousand good and evil perished then, history confirms.
Among, alas, Vanus Galerion, he who showed the way,"
To summarize the theory, the events of Seasons of the Worm Cult may be the real events re-told in the 3E 433 book
Manimarco, King of Worms. There are quite a few parallels and similarities going on with the dlc so far. Like the appearance of Lamp Knights, magical artifacts being used, the main roles Galerian and Manimarco are in, etc. If true, we could be in for a great climactic and dark ending to this year's season.
But this is just a theory, a Scrolls theory! (Sorry had to do it)
If you want to read the book,
https://en.m.uesp.net/wiki/Lore:Mannimarco,_King_of_Worms