In Book IV of his Commentaries on the Gallic War, Julius Caesar tells us all Gauls claimed descent from Dis Pater:
18. All the Gauls assert that they are descended from the god Dis, and say that this tradition has been handed down by the Druids. For that reason they compute the divisions of every season, not by the number of days, but of nights; they keep birthdays and the beginnings of months and years in such an order that the day follows the night. Among the other usages of their life, they differ in this from almost all other nations, that they do not permit their children to approach them openly until they are grown up so as to be able to bear the service of war; and they regard it as indecorous for a son of boyish age to stand in public in the presence of his father.
Gaius Julius Caesar. Commentaries on the Gallic War. Translated by W.A. McDevitte and W.S. Bohn.
I'm assuming this is an instance of interpretatio Romana, and the Gauls actually claimed descent from a chthonic Celtic god (or gods?) and not from the Roman god of the underworld. Do we know which Celtic god this might be?