He was son to Indulf but, under the tanistry system, the throne next belonged to his cousin Dubh or Duff. Cuilean, however, favoring primogeniture and father-son descent (despite that fact that his father's branch of the royal family had been junior to Dubh's for three generations), sought to seize the crown. Cuilean carried his claim to the battlefield and, defeated, fled into the night (as it were), taking with him the perjorative tag of The Whelp.
When Dubh died, murdered after 5 years on the throne, Cuilean reappeared, to lay his claim. His reign, however, was one of chaos, since he clewed to the advice of young men and to lusting after young (and not-so-young) women. He is said to have precipitated his own death, in battle, by seizing, seducing, or raping the daughter of a 'British' noble. It is not clear, in this instance, whether 'British' refers to English or native Scots and therefore whether he died in battle against invaders, or slewn by his angry countrymen.