So this is the general idea I'm going for: Myrmidia is not only the Human goddess of war but also a country and a faction within the Estalian nation.
Myrmidians have a long-lasting warrior culture and (unlike Estalians and Tileans) they also remained attached to the old ways.
By "old ways" I mean that their culture didn't change much since Myrmidia was alive back in the classic age.
They don't use black powder weapons, they are obsessed with honour, martial strength and tactics.
Of course, their religion entirely revolves around Myrmidia and his creed.
Last but not least: the country of Myrmidia is inhabited by various mythological creatures that can be tamed or killed by aspiring Myrmidian Heroes.
And that's pretty much it, here's the fictional borders of Myrmidia: https://i.imgur.com/ljfd87A.jpg

I'm not good with naming stuff so have fun replacing those few Estalian settlements with something more Greek-like.