Read "Alexandrines" at The Eldritch Dark:
This is a grim little poem from Clark Ashton Smith (CAS) using iambic lines of six feet each, as referenced in the poem's title.
These two lines from the middle stanza have a certain quality of dark clarity that only CAS could deliver:
Knowing how in the witless brains of them that were,
The drowsy, wiving worm hath prospered and hath died;
A phrase like "The drowsy, wiving worm" prompts such rich visual imagery, giving the poem a lurid animation that I've rarely experienced at the hands of any other writer.