Eushully has been, for the past quarter century, one of the constants of the VN cosmos.  Why?  Because, with their first game Ikusa Megami, they struck gold in terms of setting design.  Eroge VN/RPG's have created some seriously odd results and synergies over the past quarter century.  The company that most embodies that is Eushully and the setting that embodies it is Dir'Lifyna, the world of the Ikusa Megami series and Kamidori Alchemy Meister.   

There are lots of arguments as to why the setting was so attractive.  Ikusa Megami was not a very impressive game, even at the time.  It's visual aspects were pretty standard for turn-of-the-century VNs, (actually a bit less so, considering what the gold standard of the time was).  The gameplay was unimpressive too.  However, the story and the hints about the deep, surprisingly well thought out setting that were dropped resulted in a cult following that caused sales to erupt, resulting in the second game, which was far more refined in every way, whether it was in visuals, sound, gameplay, or even story-telling.

However, it was the telling of Celica's origin story in Ikusa Megami Zero that truly put this setting on the map and had numerous people going back to play the older games in hopes of recapturing the magic of the experience and the fascinating setting that accompanied it.  Celica's progress from an idealistic paladin of a god of adventurers to the infamous God-Slayer of the original games was harrowing, emotional, and very, very revealing about the shades of gray that compose the setting as a whole.

To really understand Dir'Lifyna and the way its stories are presented, one has to see past the limitations of the two major sides and their mutual enemy (Dark, Light, and the Old Gods).  It is very, very easy to paint the Dark as 'evil' and its denizens as corrupt and destructive.  However, in the context of Dir'Lifyna's setting, Darkness just represents one side of an eternal war for dominance over both the mortal and divine realms.  Very few of Eushully's games have a Dark-aligned god or their worshipers as the ultimate antagonist of the story, and most people aligned with the Dark seem to simply have a looser idea of what good and evil are than the more structured view of those devoted to the Light.  Throughout the Ikusa Megami series and the other side-games, Light-aligned gods and their temples are often the cause of the worst horrors one sees going on, whether it is betrayals, massacres, or endless wars. 

This doesn't mean the Light side is evil, either.  Rather, both sides have a different vision of how the world should be, and many of the Dark gods are gods who departed from the Light because their viewpoint simply was incompatible with those who led.  The god Celica originally worships, Barouhart, is a god of adventurers, a war god, and a god of storms.  He is utterly amoral in his view of the world, being pragmatic to a fault.  This is why he sacrificed the entire clergy of the region Celica was living in to corruption in order to make an attempt on the life of a single Old God who was in no way evil or attempting to disrupt the order of the world.  In his eyes, sacrificing a few thousand of his most devoted followers to seal or slay an Old God was more than a worthy exchange.

This goes to the heart of the third side of the eternal conflict of the Dir'Lifyna world, the Old Gods.  To be straight about it, the Old Gods are Earth's deities, who fought for the losing side of the War of the Corridor, during the merging of Earth and the winners' worlds into one.  Their human worshippers were stolen by the Living Gods and most of the Old Gods were killed, sealed away, or put into servitude in various ways.  The people of Dir'Lifyna generally treat the Old Gods the same way an earthling would treat one of the Cthulhu Mythos gods, with horror and fear or with a lust for power.  The gods of Dir'Lifyna treat them as potential balance-breakers in their game, seeking to eliminate their influence and relics as best they can.

Now, why are the wars of the gods reflected on the surface of the planet?  That is one of the more interesting questions.  It also feeds into the way magic works on Dir'Lifyna.  Almost all magic - except for species magic like that possessed by the Spirits and Demons - is fueled by mana.  However, most beings don't inherently produce usable mana on their own (though some sorcerers manage to do so eventually through training and study).  Most intelligent beings devote themselves to a god, their faith and raw (unusable to them) mana going to their god, who refines it for their own use before returning a portion to the worshiper.  Most of a god's power and ability to recover that power after fighting come from this act of worship.   As a result, the more worshipers offering their mana in the form of faith and prayer to them, the more power they have and the faster they can recover it.  In addition, what their followers believe about them effects the shape of their abilities, allowing them power over various aspects.  This also feeds back into what spells are easiest for their worshippers to use (Vastar's worshippers being best at using destructive Darkness Magic, Ice Magic, and Alchemy, for instance).  A worshipper of Marsterria (the antagonist deity of the Ikusa Megami series)  for instance tend to be able to use Light-based support and attack spells, but because his aspect is primarily of war, they aren't that good at healing magic (though most high-ranking priests manage to use it anyway).  On the other hand, Iryun, the goddess of healing of the Light side, has followers who are fundamentally incapable of using attack magic but are excellent at healing and support.

The moral ambiguity of all sides and the sheer complexity of the setting mean that this is a world that has endless possibilities for play, making it easily one of the most fun universes to experience in JRPGs in general, even setting aside the fact that they are eroge.