Complex sentence: Christiern Svensøn and his son Agge, who was Svend Aagesen's father, fought during the civil war following Knud Lavard's murder, on King Erik II Emune's side and Svend writes in "Brevis Historia Regum Dacie," about their participating in these battles.
Split into simpler sentences: Christiern Svensøn and his son Agge, who was Svend Aagesen's father, fought during the civil war following Knud Lavard's murder, on Erik 2. Emune side and Svend writes in "Brevis Historia Regum Dacie," about their participating in these battles.