Complex sentence: Lejre was the capital of an Iron Age kingdom, the Lejre Kingdom, which was, according to legend, ruled by kings of the Skjöldung dynasty; it is assumed to be the predecessor of medieval Denmark.
Split into simpler sentences: Lejre was the capital of an Iron Age kingdom sometimes referred to as the Lejre Kingdom. According to legend, this was ruled by kings of the Skjöldung dynasty, predecessors of the kings of medieval Denmark.