The tragic king made famous by Shakespeare first appears in Geoffrey of Monmouth’s Historia Regum Britanniae.
He succeeded his father, King Bladud, to the throne of Britain in the ninth century BC. He fathered three daughters: Goneril, Regan, and Cordelia. In trying to determine how to divide his kingdom among them, he asked each of them how much they loved him. Only Cordelia gave an honest answer, which Lear interpreted as insulting. Lear married Cordelia, without a dowry, to King Aganippus of Gaul and divided Britain between his other daughters, marrying them to regional noblemen.
Goneril, Regan, and their husbands dispossessed, ill-treated, and humiliated Lear, making him realize how foolish he had been to exile Cordelia. Eventually, he traveled to France and reconciled with his faithful daughter. Lear, Cordelia, and Aganippus roused soldiers from Gaul, led them into Britain, and reclaimed the island from the evil sisters. Lear died after three years, leaving the island to Cordelia.
See also
Leir of Britain | The Legend of King Arthur
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138