Sorry, but this post will be quite long, cause i found some material between the 100.000 letters. It,s a narration that think that maybe have not been published, is because is the transcription of a dream, a quite strangelly long and very detailed dream.
Post a file with the whole narration, that is really quite good, post here a brief, triying not to do spoiler. Anyway is far better to read the full narration that is atached to this post, is a 10 pages tale, quite good, must say.
" As far back as 450 A. D. my retrospective sense adheres altogether to Britain; but behind that point...my memory becomes Roman-The Dream... subject was one of nameless, hovering horror,· and our respective us, near the little town of Pompelo at the foot of the Pyrenees, a monstrous doom was brewing in the hills. the Strange Dark Folk (in the dream the frequently repeated phrase was Miri Nigri) who held the monstrous Sabbaths on the Kalends of Maius and November. ... authorized me to company the cohort"
As it was predictable on a HPL narration, it all finitshes in a very bad way, the narration is too much detailed to a dream.
"It was the most vivid dream I have had in a decade, and involved subconscious use of odd scraps of boyhood reading long forgotten by my waking mind. Calagurris and Pompelo are real towns of Roman Spain, now known as Calabarra and Pamplona, respectively ! ."
This narration is not a tale, he tells to a friend that is simply the transcription of a dream, explains it on a explicit way on other letter days after:Then explains ideas to turn the dream into a tipical mistery pulp narration of the 20s, ( have to say that the planed aded ideas, that dont apear on this version, were very far on narrative power from the dream itself, and maybe decreased "the vividness and significance" there is a noticeable difference of style between the dream and the planed arrangements, as if were thinked for other person.)
"As to my Roman dream-well, young man, I hate to be naive, but the fact is that most of it really was a dream, just as Ramdolph Carter and certain parts of Celephais were ! And I certainly do intend to surround it with a motivating framework and sequel (which I have already described in an epistle to Dwyer) calculated to increase its vividness and significance .. ."
Can say that the Spanish data are consistent; the small towns still exist, the geographic data involving the close presence of Pirenaic mountains is true, and even the times of travel on horse ride are correct for the distance between Pamplona and the mountains.
So, putting all toguether, I would say that this dream was not a dream at all, and that by the action of the shamans of the mountain the spirit of the questor was sent 24 centuries ago, and that magic operations let Hp with a magic capability that was reached trough some dreams, but he, even suspecting it, dont completelly realized.
(Narration and explanation data can be found in the 2nd tome of the " HPL Selected letters, 1925-1929" edited by Arkham house on 1968, narration on page 188, explanations on page 202)