I love Eushully's unique fantasy world, Dir Lifyna. Most of Eushully's games, save for a few oddball ones by the subsidiary Anastasia and Fortune Arterial, are based in this world, which began with the original Ikusa Megami (if this gets translated, somebody please smash the skull of anyone who translates the title, because they'll probably pick the worst permutation of it). The first thing that anyone going into this setting should know, if only for giggles, is that this was never intended to be an expansive setting containing ten or more games. Ikusa Megami was intended as a one-off game and was competing with Venus Blood, of all things.
However, to the people who played the game, the setting was incredibly attractive, and they sold well enough to justify a sequel, which was even more well-received (if only because the dungeon-crawler elements were toned down to normal jrpg levels).
The basic setting of the world is that, far in the past, a technologically-advanced human world created a gate/tunnel linking a world full of magic and demihumans, for reasons that pretty much boil down to boredom and stagnation as a species due to excessive technological development. Unfortunately, this accidentally caused the two worlds to begin to merge, causing a conflict between their denizens and their gods.
An important common element to note between the two worlds is that gods existed in both worlds, but the gods of the human world had mostly ceased intervening in mortal affairs openly long before, causing the near death of faith. Since faith/belief is the source of all deities' power, the humans found themselves at a surprising disadvantage in the war, because their belief in their deities was almost nonexistent. Worse, magic was quite capable of countering most of the advantages of human tech based on pure physics.
A faction of humanity chose to pursue the amalgamation of magic and tech, creating wonders and horrors (including artificial demons and gods), but over time (the war apparently lasted for generations), more and more humans switched sides, devoting themselves to gods on the other side, even as humanity's old gods were destroyed, sealed, or enslaved one by one. By the end of the war, humanity was just another race, perhaps more numerous than the others, in the service of the 'Living Gods', and the 'Old Gods' were relegated to dusty legend and actively considered evil by most, if they weren't in the service of a Living God. Human technology was, for the most part, wiped from the face of the new, merged world, and the only remnants can be found in ruins filled with monsters and/or automatic guardians.
The dominant deity of the new world is Marsterria, a minor war god who enslaved and killed more Old Gods than any other. Most of his worshipers are humans, their prolific breeding and generations of faith having given him immense power. His followers are often at odds with the protagonist of the Ikusa Megami series and nonhuman races, because of their excessive zealotry and broad determination of what species are considered 'dark races'.
Conflict between dark gods and their servants and the gods of light and theirs is a normal part of the world of Dir Lifyna, with neutral regions and nations often becoming the battlegrounds for said followers as a result. This is a world with a massive number of intelligent species, and that, in the end, is what makes it so much fun to look forward to each game, even if the flop ratio is over 50%, lol.
Damn, it was hard to do that without spoiling anything.
Edit: It should be noted that demons, angels, nagas, and a few other races were actually coexisting with humanity but hidden due to their more direct service to deities in the original human world. The nagas still maintain faith with old gods for the most part, and as a result, they are marginalized to an immense degree. Most angels 'fell' or serve one of the Living Gods now (or both), and demons are a plague, with more summoned on occasion since demon summoning was one of the few magics that remained to humanity when the worlds met.