A Drone is a nonsentient Wraith. Drones are wraiths who are attached mindlessly to the circumstances of their deaths and spend their existence re-enacting their deaths until claimed by Oblivion. The term is also used informally to refer to wraiths who are drained of their will either through Arcanoi (such as Way of the Scholar) or just steadily worn down through existence in the Underworld.
The ratio of Drones to independent wraiths is unknown, although it is implied to be high (possibly more Drones than Wraiths). The management and control of Drones is one of the major concerns that led to the creation of the Hierarchy in the first place - Drones do nothing but contribute to Oblivion, they are tightly attached to their fetters, nonsentient and eventually succumb to the void. As a result, the Hierarchy attempts to convert them into something more resistant both through soulforging and moliation - gigantic artifacts like the Sea of Souls serve as a bulwark against oblivion, but are also horrifying examples of the institutionalized brutality Stygia engages in order to survive.