Here is some additional information to be added to the Index Astartes: Sons of Horus topic:
First as the Luna Wolves and later as the Sons of Horus, the XVI 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 XVI 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.
Horus, ever a generalist and pragmatist, was prone to adopting 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 XVI 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 XVI 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.
Specialist Ranks & Formations
*The Mournival - The Mournival were Horus' most trusted advisors and confidants. Consisting of four chosen Captains of the Legion, the Mournival existed outside the rest of the XVI Legion's regular command structure. Together the Mournival functioned as the soul of the Luna Wolves, and later the Sons of Horus, supporting their Primarch and steering the Legion's temperament and decisions. The Mournival had many important responsibilities, which included making sure that the Legion and its Primarch lived up to their standards of moral excellence and the Emperor's vision of the Imperial Truth in prosecuting its duties and helping the Primarch devise strategies for more effectively carrying forth the Great Crusade.
The Mournival on occasion even provided Horus the means to posture politically as a means to save face or present a certain united front when in the presence of the XVI Legion's Imperial allies and enemies alike. Horus relied on the Mournival to balance his decisions with questions and different perspectives. In this way the four Captains served as a combination of counsellors, confidants and naysmiths. It is known that there was an element of ritual to the bond between the Mournival brothers: meetings of the Mournival took place in moonlight, a different phase of the moon was graven on the helm of each brother, and its oaths of brotherhood were made over the reflected image of a moon.
The origins of the Mournival are unclear, although there are parallels with the rites of brotherhood amongst the Jutigran clans of Terra and the ritual practice of the Selenar gene-cults of Luna, who were integral to the early integration of many Cthonian Neophytes into the XVI Legion. Whatever the Mournival's origins, its existence within the Legion certainly lies in in the early decades of the Luna Wolves' history, and it is known to have persisted for a time even after the Istvaan III Atrocity, though by then it had lost any meaningful check upon the actions of Horus and continued to exist solely due to tradition. The last members of the Mournival before the XVI Legion fell to Chaos included Captain Horus "Little Horus" Aximand (Half Moon), First Captain Ezekyle Abaddon (Full Moon), Captain Garviel Loken (New Moon) and Captain Tarik Torgaddon (Gibbous Moon).
*Justaerin - The Justaerin Terminator Squads were one of two distinct sub-formations found within the XVI Legion's elite 1st Company. Led by the Legion's First Captain, the black-armoured Justaerin were the pride of the XVI Legion. Tasked with forming the "point of the spear" of the Legion, they went where the fighting was thickest, their attack directed usually at destroying the heart of an opposing target or conducting the decapitation strike of an enemy force. Early proponents of the use of Tactical Dreadnought Armour, many of the Justaerin entered combat as Terminators, relying on the resilience this gave them to smash aside any resistance and close in for the kill.
*Catulan Reavers Assault Squads - The Catulan Reavers Assault Squads were the other sub-formation present in the XVI Legion's elite 1st Company. They too, sported the signature black armour which denoted their specialisation in harrowing actions and hit-and-run tactics. They were led by a Captain under the overall command of the Legion and the 1st Company's commanding First Captain. They also made extensive use of Terminator Armour, taking advantage of its additional protection and ability to smash aside any opposition. Used in combination, these two elite sub-formations of the 1st Company exemplified Horus' predilection for precise and overwhelming attacks against strategic targets.
*Despoiler Squads - The Tactical Squads known as Despoiler Squads were designed to be held in reserve and unleashed once a weakness in an enemy line had been identified. Trained in fire saturation techniques and carrying additional close combat weaponry for use in overrunning enemy positions, the so-called Despoiler Squads showed the dominance of the Tactical Squad in Horus' tactical thinking. Such was the effectiveness of this tactic that its use was copied in several different Legions.
*Destroyer Squads - Considered dishonourable by some Legions who made little use of them or eschewed them altogether, Destroyers were equipped with and expert in the use of otherwise proscribed and forbidden weaponry. Alongside certain factions of the Adeptus Mechanicus, only Destroyer cadres had the license to use these weapons in the forces of the Imperium by the Emperor's command. Rad-weapons, bio-alchem munitions and the burning horror of Phospex were amongst the forbidden weapons in a Destroyer Squad's dark arsenal, weapons which irrevocably tainted the ground upon which they were used. Marked by their fire-blackened and chem-scalded armour, Destroyers were often shunned and deemed somehow tainted by their fellow Battle-Brothers in many Legions and were considered at best a necessary evil, although the effectiveness of their relic-weapons in cracking especially difficult enemy defences could not be denied.
*Seeker Squads - Seeker Squads were comprised of a specialised force of Space Marines whose principal task on the battlefield was to identify an enemy's command structure, its warlords, officers, priests, demagogues, whatever form they took, and slay them with a well-placed bolt round while the battle raged round them. The Alpha Legion are said to have first crystallised the use of this tactic and squad configuration, which had with the Emperor's approval spread to the other Legions, although its use sat poorly within the combat doctrine and martial culture of some like the Space Wolves. Equipped with special issue ammunition to assist their efforts, Seeker Squads were primarily composed of the finest shots within the Legion, as a Seeker strike force often only had a small window of opportunity to take down its targets at close range, rather than enjoying the luxury of distance.