Dark Design

The Dark Design was a part of the brain in which Time Lords became "lost" when they went truly mad in a way that (PROSE: Falls the Shadow) unleashed the dark side of the mind which all Gallifreyans possessed. (PROSE: A History of the Universe, The Infinity Doctors, TV: The Ultimate Foe) Once loose, the Dark Design was like a storm across the surface of the brain. (PROSE: Falls the Shadow)

More intense than Time Lord insanity, Time Lord madness of the Dark Design could enter mental spaces no other species could fathom. (PROSE: SLEEPY) The darkness was a true form of evil, locked inside the DNA of Time Lords. While a sane Time Lord had a stabilising rational effect on space-time, a mad Time Lord might have the opposite effect of changing the past and future like a virus. (PROSE: The Infinity Doctors) Additionally, these dark sides sometimes had animalistic qualities, (TV: The Three Doctors, PROSE: The Infinity Doctors) evoking the Time Lords' relationship with the Mal'akh. (PROSE: The Gallifrey Chronicles, The Adventuress of Henrietta Street, The Book of the War)

History
Having been discovered during the telepathic Age of the Pythias, (PROSE: A History of the Universe) those that were "lost" to this condition on Gallifrey were hidden away in institutions. (PROSE: Falls the Shadow)

Within his anti-matter universe, Omega suffered from the Dark Design, (PROSE: A History of the Universe) being able to summon an embodiment of his dark side. (TV: The Three Doctors)

There was a difference between this madness and insanity, with the Seventh Doctor having encountered no sufferers of the Dark Design native to his own time, merely the insane. He knew that there were possible "cold futures" in which his own way of thinking and feeling would become alien to his present self due to the Dark Design, (PROSE: Falls the Shadow) such as the future from which the Valeyard came. (PROSE: A History of the Universe) explained the Valeyard by saying, "There is some evil in all of us, Doctor, even you. The Valeyard is an amalgamation of the darker sides of your nature..." (TV: The Ultimate Foe)

When the Seventh Doctor was trapped in a timeless mental nightmare by the telepathy virus, he wondered if he might have been inside a prison after going truly mad to the Dark Design. (PROSE: SLEEPY)

In the Infinity Doctor's reality, Omega could use the Effect to instantaneously bring out the dark side of any Time Lord. Savar VII's dark side was unlocked to serve as Omega's murderous servant on Gallifrey. Writhing within Savar's mind, this dark Savar would take the form of Savar V with shadowy empty eye sockets when it controlled their body. When Omega took the power of the Eye of Harmony away, the entire population of Gallifrey was briefly transformed into their dark selves and the planet descended into animalistic violence. (PROSE: The Infinity Doctors)

The darker side of the Eleventh Doctor's mind once manifested as the Dream Lord. (TV: Amy's Choice)