Complex sentence: Charlemagne, the King of the Franks, made a new law, the Lex Saxonum which stated that Saxons were not allowed to worship Odin (the god of the Saxons), but rather that they had to convert to Christianity on pain of death.
Split into simpler sentences: Charlemagne, the King of the Franks, made a new law, the Lex Saxonum. This law stated that Saxons were not allowed to worship Odin (the god of the Saxons), but rather that they had to convert to Christianity on pain of death.