In four pieces of flavor text among the cards of Theros - all, incidentally, common vanilla creatures - there are quotations from a book called The Theriad, apparently a work of mythological history similar to the real-world Iliad, but in prose. As far as can be gathered, the book is about a figure called "the Champion," a woman who rose to travel across Theros with a band of companions to defend it from some unknown evil. This character of Theros's past has a lot of parallels with Elspeth's role in the "present-day" Theros story. Let's take a look at the Theriad:
The first passage is from Yoked Ox. The Champion, like Elspeth, is a reluctant hero. It seems she was merely a pious farmer before she embraced her destiny, much like the hagiographic story about the Roman general Cincinnatus, who was literally summoned out of his fields to be emergency ruler of the city.
It was in fields of grain, not fields of battle, that the Champion learned to bear the yoke of duty to the gods. She worked the land long before she was called on to defend it.
It was apparently not long before the Champion and her friends encountered deadly hardship, according to the flavor text of Triton Shorethief.
At sunrise, the Champion and her companions awoke to fine their supplies gone and Brygus, their sentry, dead. Carefully arranged piles of ornamental shells gave a clear warning: go no further.
However, it is still unknown how the Champion and company overcame the suspicious and territorial Tritons - the remaining two flavor texts have to do with her relationship with the Centaurs of Theros. According to the flavor text of Pheres-Band Centaurs, she meets them en masse when she comes to Nessian Forest:
"Poets speak of your unrivaled speed," the Champion said to the assembled centaurs, "but it is plain to see that your true strength lies in your unwavering loyalty to one another."
And she quickly adds some of them to her party, one of whom gets to be the only other named character in the epic (besides poor Brygus), so says Nessian Courser's Theros printing's flavor text:
Khestes the Adamant, the Champion's closest ally among the centaurs, took one stone to his shoulder and another to his flank. He held his stride and his aim, and let fly the arrow that killed the giant Grinthax.
As the storyline of the block progresses, we'll surely see some more meaningful references to the Theriad in Born of the Gods and Journey into Nyx.