Complex sentence: The Summer Country apparently exists in the Welsh mythos independently of Alexander's fiction, as it is mentioned (if only briefly) in the ancient Welsh tale entitled' Culhwch and Olwen' as one of the places from which Arthur gathered soldiers to take to Ireland.
Split into simpler sentences: The Summer Country exists in the Welsh mythos independently of Alexander's fiction. It is mentioned in the ancient Welsh tale "Culhwch and Olwen" as one place Arthur gathered soldiers to take to Ireland.