Was Morgana in the Arthurian legends?

Yes, Fata Morgana is the Italian name of Morgan le Fey. She was Arthur's half fairy sister (fey translates as fairy). In the early versions of the Arthurian Legends she isn't much more than Arthur's half sister but as time goes on she's portrayed as a student of Merlin's hell bent on ruining people's lives. As in she alternately tries to seduce Lancelot and get him to expose his liason with Guineviere. She thwarts the Kights in many stories for apparently no reason what-so-ever. Um, once she stole Excalibur and planned to have one of her lovers kill Arthur with it but when the plan falls through she throws the scabbard into the Lake. Sometimes she's depicted as the mother of Mordred (killed Arthur-see La Morte d'Arthur) but traditionally that's Morgause's kid.

So yeah-basically she's just the stereotypical villianess: seductive, crazy, sorceress that has it out for Arthur.