Question: When was the story by Lovecraft that the Xothic legend cycle is based on first published?

Context articles: What the Moon Brings
The Crawling Chaos
Guinevere
The Call of Cthulhu
The Magical Revival
Lovecraft's Providence and Adjacent Parts
Xothic legend cycle
Medusa's Coil
The Xothic Legend Cycle: The Complete Mythos Fiction of Lin Carter
Cthulhu Mythos

"What the Moon Brings" is a short story by American horror fiction writer H. P. Lovecraft, written on June 5, 1922.
 This story was first published in "the National Amateur" in May 1923.
 It is shorter than most of Lovecraft's other short stories, and is essentially a fragment.
 It is based on one of Lovecraft's dreams (a common technique of his).
"The Crawling Chaos" is a short story by American writers H. P. Lovecraft and Winifred V. Jackson (first published April 1921 in the "United Cooperative".
 As in their other collaboration, "The Green Meadow", the tale was credited to "Elizabeth Berkeley" (Jackson) and "Lewis Theobald, Jun" (Lovecraft).
 Lovecraft wrote the entire text , but Jackson is also credited since the story was based on a dream she experienced.
Guinevere ( ; Welsh: "Gwenhwyfar" ; Breton: "Gwenivar" ), often written as Guenevere or Gwenevere, is, in Arthurian legend, the wife of King Arthur.
 She first appears as "Guanhumara" (with many spelling variants in the manuscript tradition) in Geoffrey of Monmouth's pseudo-historical chronicle of British history, the "Historia Regum Britanniae", written circa 1136 AD.
 She is also found in medieval Welsh prose, in the mid-late 12th-century tale "Culhwch and Olwen", as Arthur's wife "Gwenhwyfar".
 In medieval romances, one of the most prominent story arcs is Queen Guinevere's love affair with her husband's chief knight, Lancelot.
 This story first appeared in Chrétien de Troyes's "Lancelot, the Knight of the Cart" and became a motif in Arthurian literature, starting with the Lancelot-Grail of the early 13th century and carrying through the Post-Vulgate Cycle and Thomas Malory's "Le Morte d'Arthur".
 Guinevere and Lancelot's betrayal of Arthur preceded his eventual defeat at the Battle of Camlann by Mordred.
"The Call of Cthulhu" is a short story by the American writer H. P. Lovecraft.
 Written in the summer of 1926, it was first published in the pulp magazine "Weird Tales", in February 1928.
The Magical Revival is a book written by British occultist Kenneth Grant, first published in 1972.
 It is the first of his "Typhonian Trilogy", which comprises this work and two others—"Aleister Crowley and the Hidden God" (1973) and "Cults of the Shadow" (1975).
 In this work, he first introduced his theory that American horror author H. P. Lovecraft's Cthulhu mythos was psychic revelation presented as fiction, an idea which he would elaborate and extend further in his later works.
 This idea - that what is ostensibly presented as fiction is often a vehicle masking deeper realities - has precedence in the inclusion by Aleister Crowley of many works of fiction in the official reading syllabus of his mystical and magical order, the Argentum Astrum ; Grant's recognition of Lovecraft's value may be seen as simply a detailed continuation, an "updating" of this basic idea using material which Crowley simply hadn't had the opportunity to read.
 Grant also claimed in this work that there was an unconscious connection between Lovecraft and occultist Aleister Crowley, arguing that both of them channeled their work from the same occult forces, although Lovecraft was not consciously aware of the alleged otherworldly sources of his literary inspirations.
Lovecraft's Providence and Adjacent Parts is a book by Henry L. P. Beckwith, Jr. detailing sites in Providence, Rhode Island related to H. P. Lovecraft.
 It was first published by Donald M. Grant, Publisher, Inc. in 1979 in an edition of 1,000 copies.
 The book grew out of a bus tour of Providence that Beckwith held as part of the World Fantasy Convention.
 Sites detailed include a graveyard where Edgar Allan Poe once walked and the inspiration for Lovecraft's story "The Shunned House".
 A revised and expanded edition was published by Grant in 1986 and again in 1990 for Lovecraft's centennial.
The Xothic legend cycle is a series of short stories by Lin Carter that are based on the Cthulhu Mythos of H. P. Lovecraft, primarily on Lovecraft's stories "The Call of Cthulhu" and "Out of the Aeons".
"Medusa's Coil" is a short story by H. P. Lovecraft and Zealia Bishop.
 It was first published in "Weird Tales" magazine in January 1939, two years after Lovecraft's death.
 The story concerns the son of an American plantation owner who brings back from Paris a new wife.
 It mixes elements of Lovecraft's Cthulhu Mythos with the ancient Greek myth of Medusa, but it has also been noted for its racist aspects.
The Xothic Legend Cycle: The Complete Mythos Fiction of Lin Carter
The Cthulhu Mythos is a shared fictional universe, based on the work of American horror writer H. P. Lovecraft.
 The term was coined by August Derleth, a contemporary correspondent and protégé of Lovecraft's, to identify the settings, tropes, and lore employed by Lovecraft and his literary successors.
 The name "Cthulhu" derives from the central creature in Lovecraft's seminal short story, "The Call of Cthulhu", first published in the pulp magazine "Weird Tales" in 1926.
 Richard L. Tierney, a writer who also wrote "Mythos" tales, later applied the term "Derleth Mythos" to distinguish Lovecraft's works from Derleth's later stories, which modify key tenets of the "Mythos".
 Authors of Lovecraftian horror in particular frequently use elements of the Cthulhu Mythos.