“Go strong into the void my Sons […]”
“Rekhol – Cthonic:, Noun:, meaning: ‘They Who Run With Death’ – although a direct translation to Low Gothic is probably ‘reave’ or ‘despoil’. A ‘Rekhol’, in the Cthonic underworld, would be a warband that sets out with a specific task; be it to hunt, kill or simply raid enemy territory. As the XVI legion’s tactical structure became more fluid and ad hoc as the Heresy progressed, companies were split apart, merged or dissolved entirely as casualties and tactical necessities dictated. Some companies were split into separate ‘reaves’; sent to harry distant and crucial sub-sectors or hunt a specific prey. Some of these ‘reaves’ could be only a few squads in strength, as large as a demi-company, or as small as a single fire-team. Amongst these broken companies, many began to refer to themselves as a Rekhol – and would usually be named after the captain or champion leading it – a sign of the increasing influence of the Legion’s Cthonic heritage in their tactics and organization and a steady rejection of the tactical tenants of the Terran Legionnes Astartes.”
-Sallust the Elder, Remembrancer, 144th Expeditionary Fleet
As the Heresy wore on, the element of surprise, so carefully marshaled at Istvaan, and the early campaigns of the war were lost. The full might of the Imperium was steadily brought to bear on the traitors. On a hundred fronts, the war ground to a stalemate, made worse by the onset of the Maelstrom. Still, the two sides warred on, fighting blindly and desperately in the dark void.
In order to attempt a break in the stalemate, Horus began to dispatch smaller forces of Sons to reave behind the Imperial lines. It was hoped that these actions would divert troops from the front and cause enough widespread disruption so as to mask his Legion’s diminishing numbers. This also suited the Sons of Horus’ predatory form of warfare, letting the renegade Army units and the Legions more suited to wars of attrition – such as the Iron Warriors and Death Guard – take the brunt of the grueling and protracted campaigns.
With communication through the Maelstrom becoming less and less reliable, to the point of nonexistent, these warbands – called ‘Rekhol’ in ancient Cthonic – assumed almost complete autonomy from their parent legion, selecting their own targets, areas of operation and having responsibility for re-supplying themselves. Like the Cthonic techno-tribes they descended from, these warbands came to value their own independence and, gradually, the old Legion command structures were replaced with a brutal, and often murderous, tribal culture. While these reaving war bands proved a valuable asset for Horus; after the siege of Terra, however, it would prove a large setback for the traitors, who struggled to reunite these fractious elements without the leadership of the Warmaster. Most of these warbands would never return to the Legions’ fold, eschewing their old heraldries and loyalties to form dark and terrible legacies of their own.