The Secret Chiefs

In February 1900 Mathers accused Westcott of forging the Sprengel letters behind the Golden Dawn's charter; in 1972 Ellic Howe found 'Everything points to their being fabrications'.

Ellic Howe, The Magicians of the Golden Dawn (Routledge & Kegan Paul, 1972), chapter 'Suspect Documents'.

Hidden adepts claimed as the true authority behind the Golden Dawn, akin to Theosophy's Mahatmas.

Sources3 sources

Primary source

S. L. MacGregor Mathers, Manifesto to the Second Order, printed in Howe, The Magicians of the Golden Dawn, 1896, 29 October 1896Mathers' own account of the Secret Chiefs from whom he said he received the Second Order's wisdom, whom he met rarely in the physical body.
For my part I believe them to be human and living upon this earth; but possessing terrible superhuman powers.

Scholarship

Ellic Howe, The Magicians of the Golden Dawn: A Documentary History of a Magical Order 1887-1923, 1972, chapters 'Suspect Documents' and 'Uneasy Relationships 1893-6' (1985 Aquarian edition read)Mathers' 1900 accusation that Westcott forged the Sprengel letters, Howe's conclusion that the German letters were fabrications, and the text of Mathers' manifesto of 29 October 1896 on the Secret Chiefs.
Everything points to their being fabrications.
Vernon Harrison, 'J'Accuse: An Examination of the Hodgson Report of 1885', Journal of the Society for Psychical Research 53, no. 803, 1986, April 1986, pp. 286-310; reprinted in H. P. Blavatsky and the SPR (Theosophical University Press, 1997)The SPR's 1885 Hodgson Report judged the Mahatma Letters Blavatsky's productions; Harrison's 1986 re-examination argued Hodgson's case was one-sided and the letters not in her hand.