Complex sentence: Foreseeing Uther's death, Merlin arranges to have his sword (sometimes said to be Excalibur) magically set in a stone (or an anvil in a stone) so that only the rightful heir to the throne will be able to remove it.
Split into simpler sentences: Foreseeing Uther's death, Merlin arranges to have his sword (sometimes said to be Excalibur) set in a stone (or an anvil in a stone). Only the rightful heir to the throne will be able to remove it.