The Dark Horse is a short story written by John Goodrich. It was first published in the 2014 anthology The Dark Rites of Cthulhu, edited by Brian M. Sammons.
Description[]
In a post-apocalyptic New York where Cthulhu and his servants have awoken and subjugated the human race, a fugitive woman named Laura is approached by the mysterious Dornier who teaches her a spell which will destroy the Great Old Ones when recited by enough people at the correct time, which they will know when the sky is lit in green.
Laura spends the next few years teaching the spell to as many other survivors as she can find, before she is captured by the Lord of Manhattan, a servitor of Cthulhu. Later, while imprisoned, Laura sees the sky light up in green and begins the chant, which is taken up by all of those whom she has taught. However, Dornier has deceived them, and the spell is in fact a ritual used to summon Azathoth, and the sky opens to admit the Outer God before it reduces the Earth to ash.