Caliban was a powerful Avatara and King that taught Alric his powers. He was the one that made peace between the Fir'Bolg and the Humans before the Great War. Caliban was old and was related to Alric. He eventually was killed while trying to take back the Cath Bruig Empire. He was killed in battle against Balor himself, but not before naming Alric his successor. Alric would continue the traditions of his master and became the savior for the Light. Due to Alric's affiliation with Caliban--the Fir'Bolg honored their pact and swore fealty to Alric's cause.
It is very likely that Caliban was somehow the King of Madrigal given that his relation to Alric would allow him to ascend to the throne during the events of Myth 2 Soublighter.