Sanctuary 101 was an Adepta Sororitas Fortress-Convent on a world in Vidar Sector on the Eastern Fringes. It was the site of first contact between the Imperium and the Necrons, in 897.M41. All the Sisters inside the Santuary were killed and had their skin flayed by the Necrons. Video picts were recorded during the battle, but only showed the Sisters being cut down by flickering shadows.
"Sanctuary 101" may refer to Room 101 in the novel 1984 by George Orwell, in which the antagonist O'Brien responds, when asked what is in the room, that "The Thing in Room 101 is the worst thing in the world."