In this sonnet, Clark Ashton Smith (CAS) leans heavily on Greek mythology, so much so that it required a few visits to Wikipedia for me to catch all of the references!
What is most effective in these lines is the way that CAS uses fire and light as metaphors for romantic passion. In the first stanza, those images of illumination have a somewhat muted quality, as with "Selene's light about the Latmian boy" (i.e. moonlight shining on Endymion as he sleeps on Mount Latmus). This is continued through the end of the octet: "The spark still burning in the stoppered urn."
With the transition to the sestet, the light of love takes on a greater intensity, described as "witch-fire" so intense it "has outburned Walpurgis and the moon". The poem then closes with an indelible image as the fervor of love "lifts in quenchless rose to a cloudy noon."
Although this poem is challenging at first for a reader than does not have a full command of the legends on which it is built, it's worth the research to trace how elegantly CAS builds his blaze with those mythological elements, and then exits with an expansive vision of the power of love to overcome inclement circumstances.