Complex sentence: Saxo Grammaticus (1150--1220), also known as Saxo cognomine Longus, was a Danish historian, theologian and author, thought to have been a clerk or secretary to Absalon, Archbishop of Lund, foremost advisor to Valdemar I of Denmark.
Split into simpler sentences: Saxo Grammaticus (1150--1220), also known as Saxo cognomine Longus, was a Danish historian, theologian and author. He is thought to have been a clerk or secretary to Absalon, Archbishop of Lund, the main advisor to Valdemar I of Denmark.