Xexanoth is a fictional character from Clark Ashton Smith's Cthulhu Mythos work.
It appears only once in "The Chain of Aforgomon", where it is summoned by the main character.
Apparently, Xexanoth is the bane and mortal enemy of the time god Aforgomon and, because of Aforgomon likely being an avatar of the Outer God Yog-Sothoth, is probably an Elder or Outer God.
Dimly I heard the prompting of Atmox, and dimmer still was the sound of my own voice that made answer to Xexanoth, naming the impious necromancy which was to be effected only through its power.
As the words ceased upon my lips, I thought that darkness fluttered in the air like a great wing; and the four suns went out, and my heart was stilled as if in death.
I was still weak with the loss of blood I had yielded to Xexanoth; my senses were full of strange shadows; my servitors, coming and going about me, were as phantoms, and scarcely I heeded the pale fear in their eyes or heard the dreadful things they whispered....