Pineal gland

The Pineal Gland is a gland located inside the human brain. It has numerous biological functions (see pineal gland), but in the Middleverse, the pineal gland is connected to psychic and magical powers.

Succubi
As revealed in "The Doomsday Armageddon Apocalypse", the pineal gland is the seat of a succubus's power, and is the organ which gives her magical influence over the minds and hearts of men.

Lewis
Lewis, a fraternity pledge at Reitman University, had his pineal gland removed during brain surgery. This allowed him to see and hear ghosts.

Ivan Avi
Ivan Avi (and his mirror universe double) lost his pineal gland in a freak limbo accident at his Bar Mitzvah. This allowed him a psychic connection with his mirror universe double, and vice versa, allowing the two Ivans to communicate and pool their talents to build a quantum singularity machine.

Manservant Neville
Manservant Neville secretly harvested pineal glands from clairvoyant humans and succubi at Kerr-Avon Labs, using the manmade waterfall and lake nearby to mask his actions from the HEYDAR. Once he had assembled hundreds of them, he used them in conjunction with his Polyditetrahexamono-Trioctalon to give him the power to alter reality as he saw fit.