Abyll is Cain's father. He is only featured in the prequel un-produced Episode 0: The Alchemist as Word Paynn's only friend and confidant, much like Cain is to Moordryd Paynn. From what was released in the script to the episode The Alchemist, it seems that the relationship Abyll had with Word was identical to the one Cain had with Moordryd.
According to one of the show's creators, Abyll and his wife (parents of Cain) are still well and alive, but were never featured in the series as they disapproved of Cain's decision to leave home and join with Moordryd and his Dragon Eye Crew. Abyll also had an apparent falling-out with Word, which is why there are never seen together anymore.
His name seems to be a re-spelling of the name Abel, from the bible. This fits with his son's name Cain, although viewers don't know what this implies as to their father-son relationship, as, in the bible, Cain killed his brother Abel.