Complex sentence: Hafren was a legendary British princess who was drowned in the river Severn (;) by her repudiated stepmother Gwendolen, as told by Geoffrey of Monmouth in his pseudohistorical chronicle "Historia Regum Britanniae".
Split into simpler sentences: Hafren was a legendary British princess who was drowned in the River Severn (;) by her repudiated stepmother Gwendolen. The legend appears in Geoffrey of Monmouth's pseudohistorical chronicle "Historia Regum Britanniae".