According to the older historiography, Iljko was a duke (knez) of Croatia in 876–878 and a member of the House of Domagojević, probably the son of Domagoj. However, it has later generally been accepted that his name was a consequence of a wrong translation of the Chronicle of Dandolo, a Latin text written by Andrea Dandolo in the 14th century. During the specified timespan a son of Domagoj ruled Croatia, but his name was not preserved.