I am writing a collaboration with someone else, we want a time travel adventure, this is the background.
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On his way back from his failed crusade to Jerusalem, Richard I was kidnapped by Leopold V, Duke of Austria and Henry VI, Holy Roman Emperor. IOTL, he was ransomed for 2 year's of England's income (raised through taxing the church and peasants), but in my story there is an assassin who commits to the family of Richard I to kill both Henry and Leopold. Without England offering the ransom, Henry accepts the offer of Prince John of 60k to execute the captured king.
The tension happens between the church and the king as IOTL, but the difference is that facing the French invasion (allied by the Danes), the Emir of Cordoba accepts John's vassalage. Prince Arthur, instead of being murdered, stays in exile in France and avoids extradition and capture (narrowly), and is given the vacant title of Duke of Normandy. (All of England's land in mainland Europe is reclaimed by the French).
The assassin returns as a failure, but he makes things right by killing the tyrant King John and giving the throne to Arthur, Duke of Normandy.
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So, plausibility check -- is this possible? I still have a lot of holes to fill with this story, and a lot of figures to research. Interestingly enough it would butterfly away the magna carta.