Yxunomei isn’t just some ‘snake lady’ she’s a marilith, a powerful type of Tana'ri (demon). Here’s a little info:
‘Bold and cunning, marilith are the generals and tacticians for the Blood War. Where the balors are the influence and energy behind the war effort, the marilith are the brains and tactics. Terrible to behold, these creatures have the bodies of giant snakes, with huge, green, scaly coils. From the waist up, they are beautiful, comely females. Besides her obvious snake body, the feature that reveals the true nature of this Tana'ri is her six arms. They are commonly adorned with precious jewelry and highly decorated weapons — marilith most often carry a different weapon in each of their six hands.’
For more see here:
http://am.bigfreeweb.com/t/tanari_marilith.htm
Spoiler
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The fact that Yxunomei is a Tana’ri and the archrival of Belhifet (who is Lawful Evil) leads me to conclude that Belhifet must be a Baatezu (devil) of some kind, possibly an exiled Pit Fiend. It's at least consistant with what we know of him. The fact that he summons two Cornugon in the final battle (after HoW is installed that is) only supports this theory.
[ 05-21-2001: Message edited by: Kayless ]