Kay fairy steel defeats dragon

Thankfully, Sir Kay's brain was in rare form and notices the Dragon seems to get stronger the brighter the massive horn in its forehead glows, so Kay chucks his magical "fairy steel" spear at the horn, breaking it off.
The Dragon roars in pain and smashes into the walls, blind with terror and pain. Then it slithers into the sea while Kay stands there yelling "You better run! Camelot don't play that, homie! I’m going to pass out now, from completely unrelated stress and wounds!"

Once the Boy patches his wounds, Kay goes to check on the Mer people, and finds he done beat the dragon so bad, it's being used as a kid's playground.

Arthurian Myths Retold pg 80