Master Hillarion (also known as Hillarion Smerdis, Hilarion, and Ilarion) was a Cyprian Adept (Cyprus is the third largest and third most populous island in the Mediterranean) that the Founders met in the flesh. He wrote some occult stories with Mme. Blavatsky and also is said to be the source of some of the teachings found in Light on the Path.
the form of an Eastern adept, who has since gone for his final initiation, passing through and visiting us in his living body on his way, at Bombay.
http://www.philaletheians.co.uk/Study notes/The Masters Speak/The Oldest Armenian Monastery.pdf
Mme. Blavatsky wrote several occult stories, that after her death were published together in the book Nightmare Tales (London, New York and Madras, 1892). Boris de Zirkoff wrote:
["An Unsolved Mystery", "A Story of the Mystical", and "The Ensouled Violin"] at least, and possibly all of these stories, were written by H. P. B. in collaboration with the Cyprian Adept known as Hilarion. It is he that Master K. H. meant when, in a letter to Miss Francesca Arundale, he wrote of “the adept who writes stories with H. P. B.
Light on the Path
- Helena Petrovna Blavatsky, Collected Writings vol. VI (Wheaton, IL: Theosophical Publishing House, 1989), 355.