Talk about betrayal. Mirri Maz Duur is a priest and healer who is taken as servant/slave by Khal Drogo's khalasar. Daenerys tries to protect Mirri, and so, when Drogo needs healing, Mirri volunteers her service. That doesn't work out so well: when Drogo is dying, Mirri offers to do a bloodmagic ritual in order to save his life, even though that's not covered by his insurance. This saves Drogo's life, but not his mind or strength; oh, and it also results in the death of Dany's unborn child.
Apparently, Mirri did this all on purpose in order to get revenge on the Dothraki for burning her temple and raping her; and also to prevent Dany's son from becoming a powerful conqueror. At the end of the book, Dany ties Mirri to Drogo's funeral pyre, and her death somehow helps bring the dragons to life. Now we're talking fantasy novel.
We'll admit it: we're totally fooled by Mirri Maz Duur every time we read this book. She seems nice and convincing: how are we supposed to know that she really has this plan for revenge? And actually, her plan seems a little confusing. Did her bloodmagic ritual cause the death of Dany's unborn child? Or did it (magically) cause all of the fighting that tears the khalasar apart? Or both? And what's really important to her: that she gets revenge? Or is it more important to her to prevent future deaths by killing Dany's unborn child? Or is it a little from column A, a little from column B? Sometimes Mirri's own thoughts seem unknowable. What would it be like if she were a POV character?
We may not know what Mirri is thinking, but we sure can see her effect in the book. First, she's incredibly important for the plot: killing Drogo and giving Dany the inspiration that lets her hatch the dragon eggs.
But she also might serve as evidence that Daenerys is a little too trusting and naive. Just one more example of how Daenerys' actions may have unintended consequences (trying to save the khal ends up in his death). And, perhaps most important, she's one cause of Daenerys' transformation from trusting khaleesi who wants to save people to… whatever she'll become after this.
(Hopefully Daenerys will still want to help people, but then again, she did just burn Mirri Maz Duur alive, so we can't be sure. Let's just hope that doesn't become a habit now that she has fire-breathing dragons.)