Based in the Eye of Terror, Thousand Sons warbands are often dispatched by their daemon-primarch, Magnus the Red, to the material realm to raid, steal or otherwise enact the plans formulated by the Red Cyclops atop his tower upon the Planet of Sorcerers. While Magnus's great sorcerous Eye can discern likely targets from afar, the Thousand Sons still need a method of accurately navigating through the Warp to their locations. They do so by using Seer Stones to enhance their ability to track the psychic scent of individuals, or the sorcerous emanations of physical objects.
These Seer Stones typically appear as crystal globes around one metre in diameter, and are kept aboard Thousand Sons spacecraft. Smaller, man-portable versions exist, and are used to detect and then guide the warband to their target once they have arrived at the location.