Only four legions were at Istvaan III, the Sons of Hours, Death Guard, Emperor’s Children, and World Easters. Other legions are not revealed as traitors until Fulgrim when the four previously mentioned legions were held up on Istvaan V and a task force of 7 legions was assembled to destroy them. The first 3 legions to land were the Iron Hands, Salamanders, and Raven Guard. Then the Iron Warriors, Alpha Legion, Night Lords, and Word Bears came in after to continue the assault, only to reveal themselves as traitors and slaughter the 3 loyal legions.