Dis is the place where the Demons from Skin Deep hail from.
As of time of writing, not much is known about Dis, but it is assumed that it is inspired by The City of Dis form Dante Alighieri's The Divine Comedy, which describes the author’s trip through Hell, Purgatory, and Heaven (which take place in three parts; Inferno, Purgatorio, and Paradiso respectively.). In Inferno, Cantos 8 through 9, Dante describes Dis as a walled city surrounding the lower circles of hell, where the people who did active (rather than passive) sins are punished (circles 6-9). Dis takes its name from the roman god Dis Pater (a god of wealth, later associated with Pluto/Hades).
So far Dis has not appeared in the comic, but only has been mentioned.