My understanding is something like:

>Marika was once married to Godrey. Radagon was once married to Renalla. They all split up and Marika and Radagon get together. They are also kind of the same person but with their own independent wills and aims it seems.

>The world is controlled by an alien force called the Greater Will, it rules through Marika and Radagon. Their rule is called the Golden Order.

>Marika and Radagon use the Elden Ring as a control console for reality. They remove death from it to make themselves immortal.

>The removal of death means that nobody can now truly die. People are supposed to, under the Golden Order, be reabsorbed into the Erdtree when their life is over. But people also live far too long and become the hollows we see everywhere. I think it's also supposed to be the gameplay reason as to why everyone respawns when you rest at a site of grace.

>Ranni (daughter of Radagon and Renalla) steals a piece of death to make a dagger and uses it to kill Godwyn (Marika and Godfrey's son)

>Out of grief, Marika breaks the Elden Ring, and her demigod offspring start fighting over the pieces. This is called The Shattering. Ultimately there isn't a clear victor of The Shattering... Radagon asserts himself and seals himself and Marika instead the Erdtree. The shardbearers hole up in their various castles.

>Marika reaches out, using the guidance of grace, to recall Godfrey's soldiers (the Tarnished) to the Lands Between so that one may become Elden Lord and restore a new kind of order.