Complex sentence: The cathedral contained the black marble tomb of John Plantagenet or John Lancaster, duke of Bedford too, who is considered as Joan of Arc's murderer, he became a canon priest of the cathedral after her death.
Split into simpler sentences: The cathedral contained the black marble tomb of John Plantagenet or John Lancaster, Duke of Bedford, who is considered to be Joan of Arc's murderer. He became a canon priest of the cathedral after her death.