The Dark Design was a medical condition that certain Time Lords suffered from when they went mad. When this occurred, the Time Lord began to suffer from a storm across the surface of their brains. Those that suffered from this condition on Gallifrey were hidden away in institutions. (PROSE: Falls the Shadow)
Many inventive and brilliant Time Lords become corrupt and insane, which caused the condition of the Dark Design. Sufferers included Rassilon (AUDIO: Zagreus), Omega (TV: The Three Doctors), Morbius (TV:The Brain of Morbius) and Borusa (TV: The Five Doctors).