Schizo Lore Speculation
Let's start with what we know, Nightreign and ER share the same history up until the Shattering, so we can think of them as 2 parallel universes that split off at the event of the shattering.
This speculation is about NIGHTREIGN ONLY, NOT ELDEN RING
The "Night King"(the final final boss) has invaded the lands between, and won. The tarnished were summoned, and failed, the notable figures from the past (demigods/legends) have been killed/mind-controlled. Morgott is an example of a victim of the night king, he's actively being controlled given the purple aura of his face and how he's summoned with purple "goop".
Marika/Radagon/Elden beast were defeated but the Elden Beast survived(since this explains the guidance of grace). The last ditch effort of the Elden Beast is to summon the nightfarers, heroes from lands beyond to stop the Night King. They are called "Seekers of Redemption", Redemption has an archaic meaning of "Buying one's freedom", their lands were conquered by the Night King and they were essentially enslaved. This grants them motivation to ally with the Elden Beast.
This is what happened to Radagon:
The reason the map shifts around is because it's the final battleground between the Elden Beast and the Night King, creatures with otherworldly powers such as being able to summon characters from a different world, they are basically playing chess with us.
Finally, after defeating the Night King, our 3 champions share the power of the Elden Ring to become a council of gods for the next age. The End.
How much of what I said will be true? 0%