Gawain vs Demon Knight

Inside a beautiful woman was awaiting him and she goes "Gawain! you live! Run! The Crimson Knight isn't just a fiend! He's a literal demon!"
And Gawain's like "Neat! Gonna kill his face."
"But he's slain over 30 knights."
"RIP to them, but I'm Gawain. There's only two people who could EVER think of trying me in a fight, my king and my wife." And Gawain turns around and the Crimson Knight is there, glowing with Hellfire. The demon goes "I've built you a coffin to sleep in forever, Gawain." And Gawain's like "Awesome. FOR CAMELOT!" and clashes swords.
These people fight for TWELVE HOURS! At this point, Gawain's lost all his magical sun strength, and he's a base human and the demon's.... a demon. So, he’s looking bad, covered in his own blood, bones cracked and vision blurring. But Gawain's like "I don't need magic! I don't need super strength! I'M GONNA WIN BECAUSE I'M RIGHT! And punches the demon's helmet off, causing it to slam into one of the coffins. Then Gawain cuts his head off and goes "OK, I'mma nap in this coffin now." and falls asleep.

Arthurian Myths Retold pg 102-103