What is the discordant note heard in the end of the third stanza?

In the third stanza, the speaker calls up Xanadu along with a queer spirit, stranger than the surrounding palace, caverns, and the ocean. The speaker is overwhelmed by the images and imagines to have been turned into real and concrete. So, he cries out “Beware, Beware!” while describing the creature possessing “flashing eyes” and “floating hair”. There is dichotomy as to who this strange creature refers to. Thus, it marks the upcoming event of something sinister. This song and his vision becomes overpowering enough, so much so that the speaker turns into some “God”, consuming “honey-dew” and “the milk of Paradise”. Critics argue this image to be the resultant effect of intake of opium while others have negated this and tried to explain it as a final vision of Kubla Khan, turned into some sort of a strange, mysterious creature. Thus, an inexplicable, bizarre atmosphere of mysticism is created at the end of the third stanza.