Secret Chief

A Secret Chief (シークレットチーフ) is a type of transcendental being, spoken of in the traditions of the Golden Dawn and the Rosicrucian Order.

Although the Secret Chiefs have been referenced at points in the story, the only being seen in the series so far that has been explicitly referred to as a Secret Chief is Aiwass.

Principles
The Secret Chiefs have been described as a similar yet different sort of being from the Magic Gods (with it being noted that comparing Aiwass with Othinus could help to clarify the difference) and experts with superhuman skill. Some have claimed they guide humanity from the shadows, and give the approval for the establishment and management of all magic cabals, claims to the latter effect supposedly being made by William Wynn Westcott (with the dubious Sprengel Letters being the primary mention of Anna Sprengel and her alleged connection to the Secret Chiefs ) and Aleister Crowley (the latter with regards to Aiwass).

Samuel Liddell MacGregor Mathers claimed that the Secret Chiefs have the physical appearance of those who drank the Elixir of Life. The Secret Chiefs have also been associated with the highest levels of the Golden Dawn, namely the Third Order, though positions like 8=3 and higher have been thought of by some as made-up and just for show.

The Rosicrucian magician Anna Sprengel is a priestess of the Secret Chiefs, capable of contacting them and drawing on their power at will. Anna herself is not a Secret Chief, noting that there was no point in going to the trouble of becoming one (implying that there is a way to do so) when one could set themselves up in a position that allows free access to their power.

The sole example of a Secret Chief seen so far is Aiwass,   the entity whom Aleister Crowley contacted in 1904 and received the Book of the Law from. A being of the Aeon of Horus and an 'angel' not found in any religious or mystical text prior to that contact,  Aiwass is an energy-based entity, originating from the deepest layers of the world - at the Pure World of physical laws beyond the phases of magic and religion, and requires a vessel in order to wield its full power on the 'surface' of the world. Aiwass is commonly associated with the title Holy Guardian Angel (聖守護天使), which has occasionally been used in reference to the other Secret Chiefs. Aside from his connection to Aleister Crowley and the systems he developed, together with Anna Sprengel he is also connected to the Rosicrucian Order.

Background
Rosicrucian magician Anna Sprengel was said to be a point of contact for the Secret Chiefs and key to the founding of the Golden Dawn, though the sole mention of her within William Wynn Westcott's Sprengel Letters was considered highly dubious.

In 1904, Aleister Crowley summoned Aiwass (who had actually been summoned prior to that point by Anna Sprengel to recover her body from Madame Horos), obtaining knowledge that would be recorded in the Book of the Law.

At some point after the downfall of the original Golden Dawn, Robert William Felkin travelled across Europe in search of the Secret Chiefs, meeting a professor whose adoptive daughter claimed to be Anna Sprengel’s niece along the way.

World War III Arc
During his encounter with Fiamma of the Right, Aleister Crowley mentioned how Anna Sprengel was said to have acted as a point of contact for the Secret Chiefs in the foundation of the Golden Dawn, and he had acted as a point of contact for Aiwass.

Coronzon Arc
Anna Sprengel and her connection to the Secret Chiefs were mentioned at various points during the events surrounding Coronzon and the Ceremony of Mo Athair, during which Madame Horos emerged posing as her, and at the end of which, Aiwass dealt with the imposter and restored Anna to her stolen body.

Kamijou Arc
While walking through London with Aiwass, while information was being noted regarding the Secret Chiefs, Anna Sprengel noted that there was no point in going to the trouble of becoming one when one could set up a position for themself that allowed them to freely draw on the power of those transcendent beings, likening them to credit cards.

New Year's Arc
During his analysis of the Bridge Builders Cabal, Aleister Crowley came to the conclusion that the Transcendents were all linked to the Secret Chiefs or Holy Guardian Angels in some way - some by receiving their grimoires or knowledge, some by allowing one to reside in their body, and some by controlling them.