You are asking if Lady Macbeth is a fiend-like person in Shakespeare's Macbeth. She certainly isn't literally a fiend, nor is she a fourth witch, but I suppose you could make an argument that she is fiend-like.
She believes in what she calls "spirits" and asks them for help. In Act 1.5, she asks them, in a stern and serious manner, to "unsex" her--make her more like a man (ruthless, aggressive). She asks them to fill her with "direst cruelty." She asks them to turn her mother's milk to poison.
Later, trying to manipulate and convince Macbeth that the two of them should continue with plans to assassinate Duncan, she says that if she had promised to do something like Macbeth promised to kill Duncan (which he didn't really do, by the way), rather than go back on her word she would pluck her child from her breast while he was eating and "dashed the brains out" (Act 1.7).
Of course, an answer to your question partially depends on your definition of "fiend." But if you define fiend in a traditional or popular way, then yes you could argue that Lady Macbeth is much like a fiend.
lady macbeth can be referred to as a feind like queen because inspite of gaining the noble title of a queen, much of the origin of macbeths nemesis(deserved downfall) is based on the goading of his wife.she coaxes her husband to the extent of converting him from a noble, trustworthy warrior to a trecherous regicide. but the statement that lady macbeth is a fiend-like queen can only be used at the start of the play where she portrays her cruel nature by saying she would bash the brains out of a baby's head if needed and other such examples.
but as we reach the sleepwalking scene we realise that the once wily lady macbeth is now overpowered by her conscience leading to suicide.
hope i helped ya.