Complex sentence: Oxford joined the disaffected barons who met at Stamford and forced King John to seal Magna Carta at Runnymede on 15 June 1215, and was elected one of the 25 barons who were guarantors of the King's adherence to its terms.
Split into simpler sentences: Oxford joined the disaffected barons who met at Stamford and forced King John to issue Magna Carta at Runnymede on 15 June 1215. The earl was elected one of the barons who were to guarantee the King's adherence to its terms.