I've always loved myth and folklore so I've decided to post a series of some great characters influenced by them.
The first is Scáthach (pronounced Skaw - Thach) The Shadowy One, The Fierce Woman, She Who Strikes Fear, Scota, Scatha
Her rival, the warrior woman Aífe (Aoife is the Irish spelling), threatens her territory. Scáthach and Aife are twins. Wiki-
Also know as Lady of Shadows or Shadowy Isle she was once mortal and was touched by Tuatha de Dannan and acts like the Norse Valkyrie searching the battlefield for the souls of the slain. She initiates young men into the arts of war, as well as giving them the "friendship of her thighs", that is to say, initiating them sexually. More info here.
I first heard about Scáthach in the book Secrets of the Immortal Nicholas Flamel by Michael Scott. Scathach is also shown in World of Warcraft.
What is your favorite bad-ass female mythological character?