Queen Mab is a character portrayed by Miranda Richardson in the 1998 film Merlin. She is the primary antagonist of the story opposite Sam Neil who portrays the eponymous character.
Prior to the film's events Mab was a powerful Pagan goddess but as Christianity came on the rise man turned away from her and the "Old Ways" causing her to lose much of her power and influence.
Her ultimate agenda is therefore to eradicate the New Religion and restore mankind's belief in the Old Ways so that she and her world do not disappear forever.
At first she attempts to seek help from her sister (the Lady of the Lake - also portrayed by Richardson in the film) however her sister is unwilling to help, seeing the change of man's faith from Old Ways to New as being natural and part of a greater order.
Realizing she is alone in her intentions to restore the Old Ways she creates a half-mortal child; Merlin, who she believes will lead his fellow men back to worshipping her, thus restoring her life force and power.
Merlin is left in the care of his aunt Ambrosia whom he is clearly very close to, years later he discovers his supernatural talents and Mab returns for him with her closest disciple, the sprite; Frick. However, after living with Mab in her kingdom for a short time Merlin grows homesick and secretly departs. Mab, in an attempt to find him goes to Ambrosia's residence and subsequently (and perhaps inadvertently) kills her in a fit of rage after the old woman proves defiant and somewhat blasphemous toward her. Merlin soon arrives and as a result of the goddess's actions takes a vow to never use magic again for any purpose other than defeating her.
Later in Merlin's life Mab resurfaces, manipulating the tyrant-king Vortigen into a war with King Uther Pendragon's armies, however, as Vortigen is an atheist and does not take Mab seriously, he fails her as a warrior and Merlin slays him on the battlefield, leaving Mab without a mortal to represent her once more.
However, Mab is quick to another scheme and sways the victorious Pendragon into lusting after another man's wife, leading to yet another war. Merlin reluctantly agrees to help Uther gain the wedded woman's affections but only in return for this King's first born son (Arthur) who Merlin later trains to be a just and righteous man.
Arthur leads a Christian kingdom, much to Mab's dismay and as a result she turns to his long lost half-sister Morgan le Fay (portrayed in the film by Helena Bonham Carter) born from Arthur's mother's first marriage. Morgan is easily persuaded to aid Mab as she is born with a facial deformity which leads her to feel somewhat insecure. Morgan strikes a deal with Mab to serve her in exchange for beauty.
After being beautified Mab sends Morgan to seduce and sleep with her half-brother Arthur (who is entirely unaware of his incestuous actions). From their union comes a son; Mordred who goes on to be Mab's most formidable pawn. Morgan however does not wish to see her son be used for such purposes and attempts to stop the dying goddess - due to her disobedience Mab kills her, growing somewhat tired of mortals abandoning her.
Mordred brings war to Camelot and Arthur. In the battle's climax, Mordred and Arthur slay one another, leaving Mab, once more, alone.
Realizing she has no one left, Mab confronts Merlin herself, leading to a vicious battle of magics. However, despite being at her weakest, Mab is still a goddess and far too powerful to defeat by such means, especially as Merlin's power comes from Mab and from the Old Ways she is so desperately trying to preserve.
Ultimately, however, Merlin has an epiphany of sorts: realizing there are few Pagans left as most have been slaughtered by the invading Christian forces or have converted out of fear, Merlin sees the one true way to finish Mab is to simply cease believing in her.
He says to her, backed by his men: "we forget you Queen Mab. Go join your sister in the lake and be forgotten".
He and his mortal followers turn away from the goddess, ignoring her final desperate cries. Finally forgotten, Mab fades away and with her, the Old Ways.