Lucifer appears in Blackmore’s Prince Arthur. He supports the Saxon King Octa against Arthur, but is countered by God’s own favor for Arthur. He seems to be the Black Hermit in Perlesvaus.
There are, of course, numerous metaphorical references to Lucifer throughout the Arthurian romances.
Sources
Perlesvaus | Early 13th century
Prince Arthur: An Heroick Poem: In Ten Books | Sir Richard Blackmore, 1695