Battle of Isstvan III
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The Battle of Isstvan III was the first battle of the Horus Heresy, carried out by the Traitor Legions to purge any remaining loyalists from their ranks.
As the final step before declaring war on his father, Horus needed to get his brothers on his side and purge their legions of elements whose loyalties were uncertain. He used the secession of the Isstvan system as a cover for this activity. He, Angron, Mortarion, and Lord Commander Eidolon (Horus had sent Fulgrim to try and sway Ferrus Manus to their cause) hand-picked the squads that would form the first wave of the Legiones Astartes invasion rather than committing entire companies as was their standard operating procedure.
Once the initial wave landed, the Primarchs set about deploying virus bombs to the fleet. This was where things didn't go according to plan. Saul Tarvitz of the Emperor's Children stole a Thunderhawk and warned Nathaniel Garro aboard the Eisenstein of the Warmaster's plan. Garro's company took control of the ship, killing their brothers under the command of Ignatius Grulgor, and escaped the system after accepting refugees Iacton Qruze of the Luna Wolves, remembrancers Euphrati Keeler and Mersadie Oliton, and iterator Kyril Sindermann, who had witnessed Horus's massacre of the remembrancers and iterators in his fleet. The virus bombing exterminated all life on the surface of Isstvan III, but some Astartes found sealed bunkers and dug in. Before Horus could order a second bombing wave, Angron went down to kill them personally. Horus had to deploy the rest of his forces alongside Angron, resulting in an inefficient, bloody battle that pushed back his timetable even though it blooded the forces loyal to him, ensuring they would fight the war to their ends.
The flight of the Eisenstein allowed Nathaniel Garro to warn the Imperium of Horus's treachery. The ground battle cost Horus time that he needed to consolidate his gains and push toward Terra. Rogal Dorn organized a fleet of seven legions that would eventually reach the Isstvan system and challenge Horus in his fortified position on the Urgall Depression of Isstvan V.
Nothing can possibly go wrong.
Tabletop
Forge World's first Horus Heresy rulebook, Betrayal, is focused on this battle. In addition to the generic rules for Space Marine Legions and a Mechanicum detachment, it includes special rules for the specific Legions and characters involved, including Horus, Mortarion,Angron,Abaddon, and Garviel Loken.