Complex sentence: The preeminent pseudohistoric personage in Cornwall is King Arthur, although details concerning his life are difficult to separate from later medieval interpretation, he does seems to be based on a historical leader of the Celtic Britons.
Split into simpler sentences: The preeminent pseudohistoric personage in Cornwall is King Arthur. Although details concerning his life are difficult to separate from later medieval interpretations, he does seem to be based on a historical leader of the Celtic Britons.