The Conqueror Worm
Because it emphasizes the finality of death, it calls to question Ligeia's resurrection in the story.  Also, the inclusion of the bitter poem may have been meant to be ironic or a parody of the convention at the time, both in literature and in life.  In the mid-19th century it was common to emphasize the sacredness of death and the beauty of dying (consider Charles Dickens's Little Johnny character in Our Mutual Friend and the death of Helen Burns in Charlotte Brontë's Jane Eyre).  Instead, Ligeia speaks of fear personified in the "blood-red thing."