Studies in the fairy mythology of Arthurian romance on Morgana and Lancelot line: it's a substitute for Armor
"As soon as it became impossible by tradition for Morgain to entice the queen’s husband from her(cause sister), she naturally would be represented as endeavoring to entice the queen’s devoted lover from her. In other words, the elements of the early story are kept, though disguised, and the hero is changed. Properly Morgain’s relations to Guinevere have produced this essentially late development which reverts to the original story."
