It's kind of surprising that a character who looms so large in the "Tale" doesn't have a name. After all, she's the one responsible for sending Custance on her first rudderless boat trip (which sets a whole number of plot wheels turning). Still, she is known only as the "mooder of the Sowdan."
Well, to be fair, the narrator does launch a few choice insults her way, calling her "welle of vices," "roote of iniquitee," and "serpent under femynynytee," among other nasty things (358, 360, 364). So, it's not like she's totally without distinction.
On top of his other insults, the narrator calls the Sultan's mother a "feyned womman," implying that she is so evil that she's barely human. He could also be implying that her aggressiveness has made her barely female, since (in the narrator's view at least) all women are supposed to be just as passive as Custance. The narrator also accuses the Sultan's mother of being an instrument of Satan. He thinks the Devil has used her, just like when he manipulated Eve to bring about the Fall (a.k.a. when Eve bit the apple).
Yikes. Those are some serious accusations to be tossing out.
Okay, so impaling your own son and a bunch of Christians does sound a wee bit Satanic, but we have to cut her some slack. See, the Sultan's mother doesn't see her actions as being motivated by the Devil.
Well, she appears to truly believe that the laws of Islam are holy and given to the people by God's own prophet. She believes that the Syrians will go to Hell "for we reneyed Mahoun oure creance" (340). These are powerful motives, and ones with which Christians as well as pagans might be familiar. While the narrator sees the Sultan's mother as an evil non-human instrument of the Devil, she sees herself as an instrument of God. The apparently ruthless woman is, perhaps, humanized a bit by her faith.