Slate is the name of a populated world in the D-Verse.
The planet known to its inhabitants as Slate vaguely resembles Terra insofar as it has roughly the same gravity and climate. The landmasses are completely different, of course, having a single massive chain of islands around the equator and several much larger continents to its north and south.
Slate was initially colonized by a group of Light and Dark casters, and their supporters, who were nearly exiled from Terra before choosing to leave of their own accord. Though the ceremony to transport them failed, it attracted Eris's attention, and she opened a Gate for them as part of her plan to spread humanity across the stars.
There were seven major groups, each following one of the (self-styled) Lords; three Bright Lords who practiced Light magic, three Dark Lords who practiced Dark magic, and one Grey Lord who blended both. The six Bright and Dark Lords went their separate ways and founded six separate kingdoms across the face of the planet, while the Grey Lord stayed where he was and founded a single city, which he held to be the only true neutral ground in the worldwide ideological conflict of Light vs. Dark.
As ideologies evolved over time, the six kingdoms were surrounded by smaller, self-governing areas with varying degrees of adherence to the original philosophical duality, while the Grey Lord's neutral city remained steadfastly neutral.