When invited to follow the example of the others by his relative, the narrator refuses out of horror. As the relative tries to control one of the mounts "the suddenness of his motion dislodged the waxen mask from what should have been his head."
Out of the Tartarean leagues through which that oily river rolled uncanny, unheard, and unsuspected, there flopped rhythmically a horde of tame, trained, hybrid winged things that no sound eye could ever wholly grasp, or sound brain ever wholly remember. They were not altogether crows, nor moles, nor buzzards, nor ants, nor vampire bats, nor decomposed human beings; but something I cannot and must not recall. They flopped limply along, half with their webbed feet and half with their membraneous wings; and as they reached the throng of celebrants the cowled figures seized and mounted them, and rode off one by one along the reaches of that unlighted river, into pits and galleries of panic where poison springs feed frightful and undiscoverable cataracts.
In terror, the narrator plunges into a subterranean river and narrowly escapes with his life and sanity. While recovering in hospital, he studies the Necromonicon and discovers an answer to the mystery that is of no comfort whatsoever.
"The Festival" is a short work that demonstrates that the purpose of a short story is to deal with one "gag" and then get out. Lovecraft was inspired to write the piece after a visit to Kingsport, which at that time was notable for its antique architecture that was for Lovecraft the totality of "all the past of New England–all the past of Old England–all the past of Anglo-Saxondom and the Western World". He combined the atmosphere that he found there with his recent reading of Margaret Murray's The Witch Cult in Western Europe a work of poorly researched pseudo history about how witchcraft is the survival of pre-Christian pagan religions into modern times. Archaeologists used to pass it around for a good laugh, but it was highly influential and was one of the rickety cornerstones of the modern Wicca fads. The idea of an ancient town as the centre for the worship of some antedeluvian religion was one that Lovecraft felt would be a good vehicle for him to emulate Arthur Machen's The Novel of the Black Seal, which he greatly admired.
In the end, Lovecraft largely succeeded with a hair-raising tale that makes Shirley Jackson's "The Lottery" look like a church picnic.