From the longer Wikipedia page 
The title of Earl of Northumberland has been created several times in the Peerage of England and of Great Britain, succeeding the title Earl of Northumbria. Its most famous holders are the House of Percy (alias Perci), who were the most powerful noble family in Northern England for much of the Middle Ages. The heirs of the Percys, via a female line, were ultimately made Duke of Northumberland in 1766, and continue to hold the earldom as a subsidiary title.
The fourth Earl was involved in the political manoeuvrings of the last Yorkist kings Edward IV and Richard III. Through either indecision or treachery he did not respond in a timely manner at the Battle of Bosworth Field, and thus helped cause his ally Richard III's defeat at the hands of Henry Tudor (who became Henry VII). In 1489, he was pulled from his horse and murdered by some of his tenants.