The Hexagrammaton, colloquially known as the "Six Wings," were specialised formations that existed outside the regular chain-of-command and organisation of the Dark Angels Legion during the Great Crusade and Horus Heresy eras of the late 30th and early 31st Millennia.
These formations owed their existence both to the old Space Marine Legion formations of Terra, the amalgamation of the Terran pre-Principia Belicosa approach, the organisation of the later proscribed formations of the early Legiones Astartes, and the "Brotherhoods" of The Order from lost Caliban's knightly fellowships.
"There is a tide in the affairs of men, which taken at the flood, leads on to fortune. Omitted, all the voyage of their life is bound in shallows and in miseries. On such a full sea are we now afloat. And we must take the current when it serves, or lose our ventures."
Before the creation of the I Legion, each of the Legiones Astartes were organised instead into a formation known as the "Six Hosts of the Angels of Death," with each of these hosts having its own specialisation and purpose. During the Terran Wars of Unification the Six Hosts had each been created by the Emperor for a specific task, or from a particular type of warrior. Their names, if they had any, were unknown by the second century of the Great Crusade.
These hosts did not fight alone, being too specialised for general warfare on Terra and during the initial stages of the Great Crusade. Instead, elements of each host were combined into battle groups of different sizes and designations. As and when required, the hosts provided their troops to these armies depending upon the military need.
The warriors of each host could be called upon when a particular threat or situation presented itself. The leader of a host, elected by his sect-brothers rather than appointed by the Emperor in those times, would assume temporary command of a battle or campaign so that their expertise could be deployed fully.
It is not fully understood what happened to the Hexagrammaton in the other Legions and why its structure persisted only within the I Legion. For reasons only He knows, the Emperor chose not to continue with the Six Hosts and instead created the Principia Belicosa to structure the Legiones Astartes along the tactical lines which are now most familiar. The old ties of the Brotherhoods continued in the I Legion, though, and so the Six Hosts persisted in name and function. Being created from scratch, the later Legions were never built upon this foundation.
Following the I Legion's reunification with their Primarch Lion El'Jonson on Caliban, the Lion incorporated the traditions of the Six Hosts with those of his own native knightly brotherhood known as The Order. Thus, the Six Wings of the I Legion were created.
The "wings" of the Dark Angels existed beneath the structure of the Principia Belicosa that shaped the other Legiones Astartes, a substrate of organisation and tradition that predated the adoption of the structure of The Order of Caliban and even the amalgamation of the Six Hosts of Angels -- the Hexagrammaton -- but was also a result of both.
The I Legion did follow the principles of the later Principia Belicosa, organising their Legion's strength into Chapters, led by Masters, and Orders, which in turn were led by Paladins. These formations were further broken down into Companies, led by Captains.
Members of the Six Wings were distributed amongst squads and companies, where they acted as Captains, Sergeants, and line Legionaries, or in specialised formations when assembled on orders from their respective Voted-Lieutenant of a wing. These Voted-Lieutenants were special officers of the I Legion who acted as both commander of their wing and often served as a counsel to their Primarch.
Voted-Lieutenants were elected by their peers rather than being nominated for the rank by a superior officer. They would then have their nominated successor chosen to command the wing in the event of their untimely death, so as not to interrupt the cohesiveness of a wing's command structure.
On the field of battle, a commander responsible for an area of operations or campaign had to request the intervention of one of the Six Wings. When such a request was made and eventually accepted, the members of the requested wing were summoned from their regular duties, when possible, in order to assemble and conduct operations under the command of their Voted-Lieutenant's orders.
The Hexagrammaton formations that existed within the Dark Angels Legion at the time of the Great Crusade and Horus Heresy included the following:
The Deathwing and the Ravenwing are still used by the Dark Angels and their Unforgiven Successor Chapters to this day in their hunt for the Fallen. The other four Wings have been lost to the mists of time and are no longer fielded by the Dark Angels. They have been unseen since the time of the Horus Heresy.
The Deathwing is unique among the Space Marines Chapters in that the entire Dark Angels 1st Company fights exclusively in Terminator Armour. The Deathwing are notoriously stubborn and resistant, refusing to buckle under extreme pressure from enemy advances even when it would be more advantageous to retreat.
Yet the Deathwing is not solely a tactical formation. Promotion to the Deathwing also means that an individual is made privy to many of the Chapter's dark secrets. The most closely guarded of these secrets is that of the betrayal of Luther and the Fallen, and the subsequent loss of their Primarch, Lion El'Jonson.
The Ravenwing protocol, which focused on the use of anti-gravitic units and all-terrain bikes, had its origins in early Legiones Astartes tactics for rapid-moving search and destroy formations, some of which dated back to the nomadic war clans of pre-Unification Terra.
After Caliban's discovery during the Great Crusade, these formations were revived and expanded as they offered a form of rapid strike warfare the Knights of Caliban both understood and had perfected over many generations, though now their steeds were ones of unyielding metal, rather than flesh.
The speciality of the Dreadwing was total warfare using the most sinister of weapons. The elected Voted-Lieutenant, bearing the title of "Dreadbringer," commanded Destroyer Squads supported by other types of units to bring total annihilation of a foe on a specific battlefield.
The Dreadwing also utilised mass armoured assaults of Land Raiders, Spartan Assault Tanks, Mastodons, and Fellblades with advanced weaponry. Their hallmark was the use of dark, sinister, and shunned munitions such as Phosphex, Rad and Vortex Weapons.
The Firewing was one of the six wings of the Hexagrammaton that existed during the Great Crusade and Horus Heresy. Its purpose is not currently known, having been completely lost to the mists of time. There are, however, intriguing heraldic links between the Firewing and the sigil of the Consecrators, a Successor Chapter of the Dark Angels.
The Ironwing Protocol was developed before the Great Crusade was fully born, when the first Space Marines were used by the Emperor to exterminate threats to Mankind too dire to be allowed to endanger the glorious future.
He had designed later variants of this protocol, which sought to wed the superhuman power of the Legiones Astartes and amplify that killing strength with newly-designed, bespoke armoured units -- such as the Land Raider -- as well as mighty Dreadnoughts in close support, for the Ironwing's mission required the shock application of brute force.
Those were entered into the Principia Belicosa for use and development by the Legions that would follow, notably the Xth and IVth, but the Dark Angels would continue to develop the tactical pattern to serve their own ends.
- The Unremembered Empire (Novel) by Dan Abnett
- Angels of Caliban (Novel) by Gav Thorpe
- Warhammer Community - Secret Orders of the Dark Angels