Gnomes and underground creatures had always had little presence in the struggle for supremacy on the Main Plane.
The gnomes had once been one of the powerful races of the Main Plane as well, but during humanity's great expansion, they suffered defeat after defeat. In the end, they could no longer hold on. With the fall of their last kingdom, they withdrew sadly from the stage of contention.
At the time, most gnomes of the Main Plane departed with the Gnome Pantheon, retreating into other worlds. Only a small number remained behind, becoming wanderers across the vast land.
Some lived scattered through towns and villages, becoming ethnic minorities; others built small underground villages in the wilderness and lived in seclusion.
As for those underground creatures, there was even less to say. People mainly knew of them through tales such as "dungeon exploration"—some adventurer discovering underground ruins and finding several treasures, or some expedition discovering a group of underground Demonic Beasts, slaying them, and reaping various rewards... and so on.
Beyond such stories, the races dwelling underground were no longer familiar to the residents of the surface, as though they did not exist at all.
In truth, they had always existed. Not only did they exist, they had civilizations of their own, as well as gods of their own.
When the Orc Pantheon contacted pantheons from all sides to besiege the Human Pantheon, they naturally sought out these gods as well. Originally, those gods had not intended to join a war that meant little to them. But when they learned that the chief god of the Orc Pantheon had driven his divine kingdom straight into that of the Human Pantheon in a suicidal attack, leaving both sides grievously damaged, they changed their minds.
If Heaven bestowed an opportunity and one refused