The Royal Book of Records:
Uneasy Lies the Head that Wears the Crown - a potpourri of some of Britain's most unusual royal deaths
Edmund of East Anglia (869)
Edmund, who was later canonized as St Edmund the Martyr and is the king remembered in Bury St Edmunds, was cruelly murdered by the Danes. The Anglo-Saxon Chronicle states "here the army rode across Mercia into East Anglia, and took winter-quarters at Thetford; and that winter King Edmund fought against them, and the Danish took the victory, and killed the king and conquered all that land". The Danes went for overkill in this happenstance. Edmund was flayed; had an eagle carved into the flesh of his back; tied to a tree; shot to death with arrows; and then beheaded!
Sigurd I of Orkney (892)
This gentleman has the dubious distinction of being the only ruler known to have been bitten to death....by the dead! Sigurd "the Mighty" was a leader in the Viking conquest of much of northern Scotland. According to the Orkneyinga Saga, following his defeat of Mael Brigte "the Bucktoothed" of Moray, Sigurd strapped his opponent's decapitated head to his saddle as a sign of his triumph. As he rode, however, Mael Brigte's buck teeth rubbed against Sigurd's leg, causing a wound that turned septic and taking Sigurd out through blood poisoning! Guess Mael Brigte ultimately won that contest, hmmm?
Edmund II of England (1016)
Edmund was the son of King Ethelred "the Unready" and attempted to hold back a Viking invasion after his father's demise. This bit of royal death fun was recorded by an uncited chronicler, disputing Edmund Ironside's death of supposed natural causes in London soon after signing a peace treaty with Canute (in which the English got to keep Wessex whilst ceding control of Northumbria and Mercia to the Danes). Allegedly an assassin hid under the king's privy, and when Nature took its course, stabbed the king twice up through the bowels. Edmund is the only ruler believed to have been murdered on the toilet! (though George II did have a heart attack whilst visiting Mr. Crapper and subsequently demised after being removed from the royal close stool)
Harald I of Orkney (1131)
Harald was co-earl with his half-brother, Paul. Harald's mother and aunt plotted to get rid of Paul and made a poisoned shirt. Harald's sibling rivalry roared to the fore when he saw that his brother had finer raiment than he, and he donned the garment. According to the Orkneyinga Saga, "his flesh started to quiver & he began to suffer terrible pain. He had to go to bed and it was not long before he was dead". Ooops! (You can't make this stuff up.)
Edward II of England (1327)
Recorded in the Holinshed Chronicle and gleefully whispered about by British schoolboys to this day, the story of how Queen Isabella (AKA "the She-Wolf of France") and her lover Roger Mortimer disposed of her unwanted spouse is a classic. Edward had male favorites, Piers Gaveston and Hugh Despenser, and it was twitted about (before there was Twitter, even) that the king preferred men to women. Supposedly Edward, held prisoner in Berkeley Castle in Gloucestershire after being forced to abdicate in favor of his 15-year-old son (Edward III), was disembowelled! This was allegedly accomplished by a horn being inserted through his anus through which a red-hot spit was inserted and wriggled about until he was dead. Ouch, that's gotta hurt!
source: British Kings & Queens: The Complete Biographical Encyclopedia of the Kings & Queens of Great Britain - author: Mike Ashley Publisher: Barnes & Noble Books (2000) ISBN-10: 0760720347 ISBN-13: 978-0760720349
[paraphrased by my good self to be far more entertaining than the original, natch]
Onward to Part II!