A Helm Mechanicum, also called a Helm Mechanicus, is a type of mind impulse technology used by lesser Nobles to pilot the smallest class of Imperial Knights known as Knights Armiger, which include the Armiger Warglaive and the Armiger Helverin. A Helm Mechanicum is similar in function to the more advanced Throne Mechanicum required to pilot the larger variants of Knight and allows the Noble to merge his mind with the Armiger's mechanisms such that they can fight as almost a single organism. Like the Throne Mechanicum, the Helm Mechanicum retains a copy of the psyche and memories or "ghost" of each of its users.
Armigers are usually piloted by the lesser Nobles of each Knight household known as Bondsmen; those whose lineage is not directly linked to the lords of the court. In taking on the role of Armiger pilot, they fulfil a solemn vow, donning the fabled Helm Mechanicum with which the spirit of the man or woman and their great war machine is linked into a single entity.
The Helm is linked by invisible chains of data to the Throne Mechanicum of their Noble liege's larger Knight armour -- chains so strong they can be plucked as the strings of a puppet by the Noble within the Throne Mechanicum in times of great need. This forces the Armiger pilots to carry out the actions directed by their liege, as the higher-ranking Noble takes over control of his vassal Knights' basic functions.
- Codex: Imperial Knights (8th Edition), pp. 16, 58-59
- Warhammer 40,000: Forgebane (8th Edition) (Boxed Set Book), pg. 11