This is a Warhammer fan-fiction work.
The Battle of Isstvan V, also known as the Drop Site Massacre, was one of the most decisive battles in the early stages of the Horus Heresy in Warhammer 40K. Before this great battle, Warmaster Horus had purged the loyalists in his own Legion and three others with virus bombs on Isstvan III. He then openly raised the banner of rebellion on Isstvan V, leading four Legions: the Iron Warriors, Night Lords, Alpha Legion, and Word Bearers.
The Emperor was enraged. Around 006.M31, he dispatched seven loyalist Legions to put down the rebellion. The Iron Hands, Salamanders, and Raven Guard led the assault, making the first orbital drop, while the remaining Legions, including the Ultramarines, were unable to arrive in time due to factors such as warp storms. The brutal battle ended in a crushing defeat for the loyalists. More than 250,000 loyalist Adeptus Astartes fell, and Ferrus Manus, Primarch of the Iron Hands, was slain by his former close friend Fulgrim, Primarch of the Emperor's Children.
The main force of the Ultramarines was delayed by warp storms
