The Drop Site Massacre

Isstvan V is the benchmark of operational betrayal. The geometry: in the Urgall Depression the first wave (X, XVIII, XIX) engages the XVI, III, XII and XIV; the second wave (XVII, IV, VIII, XX) takes the rim as “support” and opens fire in the back. After the reveal the ratio is on the order of eight Legions to three, plus the Mortis Titans. The canon insists on psychology: Ferrus does not wait for full concentration; Corax sees the trap earlier but cannot get his brothers off the hook; Vulkan holds the centre like a shield. First-wave figures according to Journal Tactica: the X 10,000, the XVIII 83,000, the XIX 79,000, in all 172,000 loyalist Astartes. About 8,000 survived (as calculated from Journal Tactica): 734 of the Iron Hands, 2,556 of the Salamanders (a disputed figure: Fandom gives only losses of “up to 98%”), 4,724 of the Raven Guard (a disputed figure: in the audio drama “Raven’s Flight” the survivors are “less than three thousand”, in retellings of “Deliverance Lost” about three or five thousand leave with Corax — we do not smooth the discrepancy over). With this Horus buys seven years during which Terra will not receive three Legions.

Loyalist first wave: the X (the elite of the Morlocks + clans), the XVIII (almost the whole Legion), the XIX (the bulk). The open traitors at the bottom: the XVI, III, XII, XIV. Second wave: the XVII, IV, VIII, XX. Titans: Legio Mortis. The traitor fleet holds the orbit.

Horus’s strategic peak before the Siege: from here on he wins time and worlds, but he will not again destroy loyalist armies on such a scale in a single blow.