Black Legion - Pre-Heresy: First as the Luna Wolves and later as the Sons of Horus, the XVIth Legion maintained much of their Legion structure as it had existed since the wars for Terra and Solar Unity, which had adhered closely to the Terran pattern laid down by the Imperial Officio Militaris before the Space Marine Legions' unification with their Primarchs.The smallest formation within the Logos Terra Militia, and therefore within the Luna Wolves, was the squad. This consisted of a group of Luna Wolves under the command of a Sergeant. Squads varied widely in both size and specialisation, with the majority of the units ranging between 10 to 20 Space Marines within the XVIth Legion. Conversely, very specialised squads such as reconnaissance units or those that might have suffered heavy casualties, might only consist of a handful of Space Marines in active service.The specialties and wargear of squads in the Luna Wolves included all of those generally found in other Legions, including those designated Destroyer and Seeker units, who were shunned by some and existed in lesser quantities. The Luna Wolves also maintained considerable resources in terms of armour and vehicles, with tithed industrial worlds they had brought into Imperial Compliance and good relations with the Mechanicum ensuring a steady supply of munitions and materials for the XVIth Legion, which spent such political coin readily.Even Horus, a generalist and a pragmatist, upon being given command of his Legion is thought to have adopted new squad formations after having seen their effectiveness in other Legions. For example, it is noticeable that until the Lactrical Onslaught the number of Storm Shield-equipped Breacher Squads was relatively low in the XVIth Legion. The contributions of such units amongst the forces of the Imperial Fists cannot have escaped his notice.After the annihilation of the Lactrical, the number of such squads in the ranks of the Luna Wolves rose noticeably. Other anecdotal evidence of this adaptability and willingness to embrace new weapons of warfare can be found in Horus' vocal backing of the Tactical Dreadnought Armour project, with the result that his Legion was one of the first and most widely equipped with Terminator Armour and at the forefront of the development of tactics for its use in assaults.The XVIth Legion showed a preference for the use of Tactical Squads. Squads configured in this form within the Luna Wolves and later the Sons of Horus outnumbered all other squad types combined throughout the Great Crusade. Horus remarked on several occasions that there were few challenges of war that could not be met by, or did not require, the use of such units.The presence of Tactical Squads designed to be held in reserve and unleashed once a weakness in an enemy line had been identified, trained in fire saturation and carrying additional close combat weaponry for use in overrunning enemy positions -- the so-called Despoiler Squads -- shows again the dominance of the place of the Tactical Squad in Horus' tactical thinking. Such was the effectiveness of this tactic that its use was copied by several different Legions, such as the White Scars and Iron Warriors, who field Despoiler Squads of their own.The heart of the Sons of Horus Legion was the company, which served as the Legion's principal military division. Made up of a grouping of squads under the leadership of an officer with the rank of Captain, the company was the base currency of campaigns and battles. There was no fixed strength for a company within the Luna Wolves and the Sons of Horus.While other Legions codified and enforced strict limits on the size of similar formations, this was not the case amongst the Sons of Horus, which had begun in more regimented form, but had become increasingly ad hoc in structure and disposition over time. Company strengths as small as 36 and as large 972 Astartes were recorded by datafactors during the XVIth Legion's action against the Dasim Patrimony, for example.The configuration of squad types within a company varied as widely as its strength. Some were comprised almost exclusively of Tactical Squads with a few Support Squads. Others were an eclectic mix based on the varied requirements of different campaigns and the will of Horus. By way of example, the 17th Company, known as the Hesperus Guard, had a standard strength of 205 Space Marines at the time of the virus-bombing of Istvaan III. Tactical Squads made up half this strength numerically, with the Legion placing in general great importance on the use of Tactical Squads in every deployment.The rest consisted of two veteran units, three reconnaissance units, a heavy support squad and multiple batteries of support weaponry. The elite 1st Company showed even greater variation. Small in number, it contained two distinct sub-formations: the Justaerin Terminator Squads and the Catulan Reaver Assault Squads. Both sported the black armour worn only by this elite company and each was led by a Captain under the overall command of the Legion's First Captain, Ezekyle Abaddon. Used in combination, the 1st Company exemplified Horus' predilection for precise and overwhelming attacks against strategic targets.In other Space Marine Legions a company would form part of further layers of hierarchical military organisation, variously referred to as battalions, cohorts, chapters, regiments or by any number of other titles. Horus seems to have preferred to avoid this extra layer of fixed organisation, which eroded over time in the Legion and was largely academic by the time of their transition to become the Sons of Horus.Instead of a formal structure, Horus would group companies and individual units together as required for the execution of a particular campaign. The commander of such a formation would usually be a senior Captain. If the formation was especially large, then other Captains would take on the role of lieutenants to the overall force commander until the completion of the campaign. These formations rarely had formal titles, but the Sons of Horus commonly referred to formations intended to prosecute a rapid assault as "Speartips."In eschewing formality and fixed structure above the basic level of the company, Horus demonstrated his pragmatism and his preference for waging war with careful precision. Within the Luna Wolves and the Sons of Horus, squads also commonly had their own honorific or epithetic titles rather than simple numerations: the Illuminators Prime, Death Makers, Jerrok's Reavers, the First Sons, and similar appelations, while some were named for the Sergeant or Chieftain that led them if their leader's own reputation was strong enough alone.Many of these titles betrayed the culture of Cthonian gang honours and the tradition of reputation and internecine warfare from which they had sprung. This culture had grown steadily stronger over the years within the rank and file of the XVIth Legion's intake of Neophytes.The exact disposition of the Sons of Horus at the time of the Istvaan III Atrocity is uncertain. Given the accounts of the battle on Istvaan III following Horus' treacherous bombardment from those that survived it, it would seem likely that the culled Loyalist elements of the XVIth Legion represented something approaching a third of the Sons of Horus' entire force.Records, tainted as they may be, place the Sons of Horus at a fighting strength of approximately 130,000-170,000 Space Marines in the period leading up to the Istvaan III Atrocity. Although the figure may have been higher, this estimate would also tally with more general assessments of the Sons of Horus Legion being in the upper quarter of the Legions in terms of the Space Marine manpower available to them.The XVIth Legion's Great Crusade fleet was likewise accorded to be among the greatest under any single commander's flag, with in excess of one hundred capital ships and perhaps three times that figure in smaller Cruisers and Escorts under Horus' direct command. Taking into account likely losses from the ground war that followed the virus-bombing of Istvaan III, and elements of the Sons of Horus Legion not in the Istvaan System at the time, it would follow that Horus began his war of betrayal with around 70,000-110,000 Space Marines of his own Legion at his disposal, with considerable evidence present in Imperial records that the latter figure is the more accurate.Heavily engaged on the surface of Istvaan III during the Istvaan Atrocity, it is estimated that some 30,000 of the Sons of Horus' Legionaries were dead or unaccounted for in the aftermath of that battle. It has been theorised by some analysts that as a consequence of the initial difficulties encountered with the purging of the Terrans from the Legion's ranks, and the unexpected cost in lives and materiel the battle had entailed, Horus was deliberately cautious with the use of his Legion during the later Drop Site Massacre.In this it is thought that he hoped perhaps to preserve as much of the Sons of Horus' strength as possible for the intended push on Terra that was planned to follow, or perhaps he simply meant to have others blood themselves in his cause while destroying the Loyalists.