Complex sentence: Word reaches Ragnar of Jarl Borg's treachery and he abandons the campaign, sailing off in four of the Viking longships, two of which sink on the way back to Scandinavia.
Split into simpler sentences: Word reaches Ragnar of Jarl Borg's treachery. He abandons the campaign, sailing off in four of the Viking longships, two of which sink on the way back to Scandinavia.