In Theros block, it seems, every vanilla creature's flavor text has to conform to a particular rule - those cards, and only those cards, have flavor text that quotes an epic poem from an anonymous author, The Theriad. This tale refers to the adventures of a young woman who long ago was summoned by Heliod and sent on a quest - much like the Sun God has done to Elspeth during the events of this block. Though there were obviously many more vanillas in Theros than there are in Born of the Gods - the former is a large set and all - there are still three creatures without any rules text in Born of the Gods, all of which have new Theriad passages for flavor text.
Swordwise Centaur, the green vanilla and Limited star of the bunch, seems to be the first passage chronologically in the story, as our hero isn't even a Champion yet, but just a little girl training to be a warrior under a Chiron-esque tutor:
The girl who would become the Champion of the Sun hacked furiously at the practice dummy. At last she stopped, breathing heavily, and looked up at her instructor.
"So much anger," said the centaur. "I will teach you the ways of war, child. But first you must make peace with yourself."
Next up, Great Hart, which seems to be a reference to the golden hind, sacred to Artemis, that Hercules had to catch but not kill on one of his Labors, tells the story of how the original Solar Champion was recognized:
The great hart stood like a statue, its hide painted gold by the dawn. The Champion laid down her weapons and stepped forward within an arm's length of the beast. The hart, sacred to Heliod and bathed in the god's own light, bowed to the Champion, marking her as the Chosen of the Sun God.
Finally, Cyclops of One-Eyed Pass shows the first Champion doing her best impression of Odysseus in Polyphemus's cave (and presumably the setup for one of the most flavorful Magic cards ever made, Eye Gouge):
The Champion armed herself to face the cyclops, heedless of her companions' despair.
"How will you defeat it with only one spear?" asked young Althemone.
The Champion raised her weapon. "It has but one eye."
Keep in mind that we're getting these lines out of order, and since there are not nearly enough vanillas in the block to write a whole epic on, we'll definitely only see parts of the story. Still, it's fascinating to see so many of the Greek epic tropes given a Magic twist in the tale-within-a-tale, and we can count on at least a couple more of these in Journey into Nyx, paralleling the end of Elspeth's trials on Theros.