What real life person, persons, or events was Morgan le Faye, cast as an evil enchantress in most King Arthur tales, based on?
It seems that she is a stereotypical fictional character, mainly a female sorcerer or goddess, based on early Celtic and Welsh mythology, poems and legends. The character isn't based around any particular person from what I understand.
You can find more info about the character here: Morgan le Fay
Though I lack a link for you, my reading of grail histories has always implied that Morgan was a characterization of Lilith.
Interestingly, the Old English word 'morgan' derives from the Germanic, relating both to "morning", and to "tomorrow". As such, 'morgen' is possibly a light-bringer meaning - which would parallel such name with the strengths gained by those who overcome one who has such name (e.g. Lucifer).
p.s.; (quaintly enough, the Dutch use the same work as a unit of measure, based upon the rough amount of land that one can farm during the length-of-time that is a 'morning').