Sothis, the fell star came from possibly space, put at end all the sithers wars (they were technologically advanced to modern times) and restored the land and went to rest (like Byleth rested for 5 years after being wounded in the attack to the monastery)
Nemesis sneaked into her tomb, killed Sothis, made a sword out of her bones, got the crest of flames from her blood and massacred her children and made more weapons out of the bodies of those massacred and gave the crests to 12 other warriors, the elites.
Rhea, who survived the massacre, helped found the Adrestrian Empire and gave her blood to the first emperor, took the name Serios, and led adrestia to hunt down the elites and nemesis. They pardoned the families of the elites, gave them positions of power and created the church with the myth that Sothis was a god and she gifted the crests and weapons to the elites.
(This is also what Rhea talks about when facing Edelgard. You humans betrayed me and my mother! It refers to those events back then.)
In golden deer, after killing edelgard, you get a letter from hubert were he gives you the intel of the sithers, Claude confronts Rhea and she tells the whole story. They go find the sithers, Rhea nearly sacrifices herself for the party and finallly you face Nemesis and the 12 elites resurrected.
They paint Rhea as someome who is trying to stop the whole thing from happening again. Specially the hunt of her and the few surviving nabeteans (descendants of sothis). After playing all routes, it feels like if they all had sat to talk at ch1, the whole thing could have been avoided and we would have gotten a golden ending.