666 of Hell

In ancient lore it is said that the 666 of Hell would wage war on other planes and were unable to ever be stopped once they gained a foothold due to their organization and military might. Eventually they made their way into the Material Plane and rampaged across Tomeria.
After some time they were repulsed back into their original planes with all major portals to the Material Plane sealed and fallen out of lore. There are still small portals that remain and summoning circles by evil magic users that bring devils of varying sorts into the material plane still which sporadically wreak havoc among the lands.