Complex sentence: In 865/866 it escalated further on the arrival of what the Saxons called the Great Heathen Army, the size of which has been estimated at between five hundred and a thousand men which was under the leadership of the brothers Ivar the Boneless, Ubbe and Halfdan Ragnarsson.
Split into simpler sentences: In 865/866 it escalated further on the arrival of what the Saxons called the Great Heathen Army. It was said to have been under the leadership of the brothers Ivar the Boneless, Ubbe and Halfdan Ragnarsson.