Complex sentence: Sauron entered the Greenwood around and built his fortress Dol Guldur near the Anduin in the southern reaches of the great wood, which began its descent into blackness and evil, eventually becoming known as Mirkwood.
Split into simpler sentences: Sauron entered the Greenwood around and built his fortress Dol Guldur near the Anduin in the southern reaches of the great wood. From this time the Greenwood began its descent into blackness and evil, eventually becoming known as Mirkwood.