Haemonculi are Eldar by species, but appear markedly different to their Dark Eldar brethren. They display physical characteristics suggestive of old age or disease: corpse-like features, sunken eyes, parchment-like skin stained with blemishes and scars, and often signs of crude surgery. Despite their apparent frailty Haemonculi are often more resistant to physical damage than other Eldar. Many, particularly those that appear influential and prosperous, make use of implanted medical devices, the exact purposes of which are unknown.

Haemonculi live in Commorragh, reportedly residing in laboratories provided by their sponsors and benefactors. Rumours suggest that some have constructed laboratories of their own, allowing them to operate as they see fit, tied to no particular Kabal, but most appear to make use of their employers' facilities, taking only their knowledge and servants with them when they decide to serve a different master.

Though not warriors by nature, Haemonculi often accompany Dark Eldar raiding parties in an advisory role, and to oversee their charges, the Grotesques and Talos, in battle. First contact is subject to the same historical uncertainties as that with the Dark Eldar race itself.

Haemonculi are, as mentioned, physically resilient, and are capable of holding their own in combat. Also, they generally make use of a variety of weapons of their own devising, generally based around poisons and biological toxins. The effects of these weapons varies depending on the skill of the maker, but they can be hideously destructive. The visible suffering and agony of their victims is a constant source of demoralisation for their enemies.

Haemonculi are torturers by trade, and have refined all the technological aids of this practice to the point of art form. In other areas they have access to, and knowledge of, almost all Dark Eldar technologies, but they are true masters of the blade and scalpel, toxin and venom. Their abilities also extend to cybernetics, psychosurgery and some forms of psychic manipulation, though they, like all Dark Eldar, are not psychically-endowed themselves. The pinnacle of the Haemonculus's art is the Talos, a great, vicious war machine built around a living victim whose ceaseless agony endows the construction with a unique will to destroy. Its weapons include powerful blades, capable of shattering personal armour and vehicle plating, and an extremely rapid firing variant on the common Dark Eldar splinter cannon. Most Talos seem to be constructed around a common basic frame, but Haemonculi seem to enjoy adding their own unique touches to these machines.

The Imperium considers Haemonculi to be a significant contributor to the threat posed by the Dark Eldar. Unfortunately, due to their position in Dark Eldar society, it seems impossible to isolate and eliminate them as a group, short of the annihilation of Commorragh itself. Nevertheless, Imperial agents and assassins are aware of Haemonculi, and will prioritise them as targets accordingly: the loss of an Haemonculus can go some way to reducing the battle effectiveness of a raiding party.

Haemonculi are hired specialists within Commorragh, but it is understood that some areas of their art related to the treatment of prisoners are considered essential by all Dark Eldar Lords, so only a very poor Kabal would be without the services of at least one Haemonculus. Skilled Haemonculi are in high demand in Commorragh, such that the best enjoy a privileged position and can essentially choose their employers. Within themselves Haemonculi have no clearly defined system of ranks, and their hierarchy, such as it is, is based on skill alone.

As noted, Haemonculi are highly individual in their armament and appearance, although all share the corpse-like appearance which seems a by-product of their art. It should be noted here that Haemonculi, more so even than other Dark Eldar, are self-serving by nature, and will not hesitate to betray or abandon their comrades if they consider it to be to their advantage.