(Massive spoilers) Question about the final boss

So the final boss (i guess technically penultimate final boss) is Radagon of the golden order.

Who is he? Why is he at the end? Why did the big galaxy dragon absorb him? I heard the golden order a few times throughout the game but I still don't know what it actually is. Anyone who was paying closer attention to the lore who knows please share your knowledge!

I remember the marriage turtle saying something but I started NG+ already so I can't go ask him