Complex sentence: Eric's succession was one in a line of successions which did not precisely follow the laws of inheritance, but excluded one or a few undesirable heirs, leading to Norway formally becoming an elective kingdom in 1450.
Split into simpler sentences: Eric's succession was one in a line of successions which did not precisely follow the laws of inheritance, but excluded one or a few undesirable heirs. Leading to Norway formally becoming an elective kingdom in 1450.