Complex sentence: Sigebert of Gembloux (c.1030--October 5,1112), medieval author, mainly historian, became in early life a monk in the Benedictine abbey of Gembloux.
Split into simpler sentences: Sigebert of Gembloux (Sigebertus Gemblacensis) (c.1030--October 5,1112), was a medieval author, known mainly as a historian. He became in early life a monk in the Benedictine abbey of Gembloux.