Arthur vs Arthurian

Arthur is a noun but Arthurian is not a noun.

Word	Noun	Adjective	Verb	Adverb
Arthur	Yes	No	No	No
Arthurian	No	Yes	No	No

Arthur (noun)	Arthurian (noun)
a legendary king of the Britons (possibly based on a historical figure in the 6th century but the story has been retold too many times to be sure); said to have led the Knights of the Round Table at Camelot
elected vice president and became 21st President of the United States when Garfield was assassinated (1830-1886)

Arthur (adjective)	Arthurian (adjective)
	of or relating to King Arthur and the Knights of the Round Table