Complex sentence: As an act of revenge after her son proclaimed himself King of Sweden, the King of Denmark forced her to sew a sack; after she was finished, he allegedly had her placed in the sack and thrown in the sea, where she drowned.
Split into simpler sentences: As an act of revenge after her son proclaimed himself King of Sweden, the King of Denmark forced her to sew a sack. After she was finished, he allegedly had her placed in the sack and have it thrown in the sea, where she drowned.