Consumed by the darkness provided by Avenger (who was thought to be Angara Mainyu at the time), Dark Saber is the servant of Mato Sakura. Her original Master, Emiya Shiro, later had to make the bitter choice of killing Saber in order to save her from her misery; a fate she gladly accepted.
Beautiful costume! I found this on Flickr the other day, and the cosplayer’s name is Calssara there. Most of her photos had descriptions saying the costume is for a German Convention, so maybe she’s German? Either way, she makes a great Saber! Good job!