Died from poisoning is in Murder.
His Matilda Unknown Baroness Mortimer was suspected. She sought the protection of Margaret of France Queen Consort England (age 25). After the death of King Edward "Longshanks" I of England (age 65) in 1307 she, Matilda Unknown Baroness Mortimer died suddenly, probabnly murdered in revenge for the murder of her husband.
On 07 Oct 1368 Lionel Plantagenet 1st Duke of Clarence (age 29) died from poisoning at Alba [Map]. Duke Clarence 1C 1362 extinct. There was strong speculation he had been poisoned by his wife's (age 14) father (age 48). He was buried at Clare Priory [Map]. His daughter Philippa Plantagenet Countess March 5th Countess Ulster (age 13) succeeded 5th Countess Ulster.
In 1425 John "Pitiless" Wittelsbach II Duke Bavaria Straubing (age 50) died from poisoning. His uncle Otto "The Bavarian" Wittelsbach V Duke Bavaria succeeded V Duke Bavaria.