Looks like I answered my own question. With a little looking, I found this excerpt and it's what I was remembering:
More at the link, I believe I'm not supposed to quote too much from outside sources:
"Hours passed slowly by and in the increasing cold of the chamber Bruton began to feel that there were hostile forces around him, eerie shadows
crowded in on him from all sides and a dark apparition advanced menacingly towards him. All the legends of evil ghosts haunting the areas around the
pyramids told to him by Arab villagers came to his mind. Soon there was a circle of antagonistic beings surrounding him, 'monstrous elemental
creations, evil horrors of the underworld' as he described them. Despite being seized by panic Brunton resisted the temptation to turn on the
flashlight and decided to see it through. It was this determination that saved him. The elementals disappeared quite suddenly, and all was dark and
Soon afterwards phantoms of a different aspect appeared - benevolent beings in the ceremonial dress of ancient Egyptian high priests. The ancient
apparitions led him through secret passages within the Pyramid to a Hall of Learning, equating to a mystical journey within the mind.
This mystical experience apparently also involved astral projection. Brunton describes feeling paralyzed, his own spirit leaving his physical body and
going to the 'regions beyond death'. He felt as if he were passing upwards through a narrow hole (also described in shamanic tradition) and becoming
a pure mental being with a sense of existence many times more vivid than when in his physical body. He described gazing down on his prone lifeless
form and noted a train of faint silvery light projecting itself from his spiritual to his physical body. This 'cord' is mentioned by many people who
have experienced astral projection. He was given various information by his ancient guides, including the fact that the Pyramid was built in the time
of Atlantis, and that the Pyramid's secret chambers and ancient records were all contained within himself.
He returned to normal consciousness quite suddenly, convinced that he had undergone some kind of initiation ceremony. Interestingly enough, it was the
belief of occultists like Madame Blavatsky that priests of the Egyptian Mysteries used the Great Pyramid to initiate candidates into the experience of
astral projection and the truth of life after death."