Mongisard became a chaos cult's base of operations on Stygia XII. There they began conducting a ritual to open a gate to the warp, which brought them to the attention of Inquisitor Abraham Vinculus of the Ordo Hereticus. Under his orders Interrogator Edwin Savaul besieged the keep, with forces from the Inquisitorial Stormtroopers and Adepta Sororitas, led by Helena. If they were unable to stop the ritaul then the planet would have be subjected to an exterminatus. They attacked the keep's doors for a week without making progress, and the end of each day brought the ritual closer to completion. It was during this time, that Castellan Marius Reinhart of the Black Templars' Sword Brethren, landed on the planet and met with the Interrogator. Reinhart came under orders from High Marshal Ludoldus to reclaim Montgisard, but after hearing of the ritual both sides agreed to work together to destroy the keep.
Using a secret tunnel the Black Templars, Savaul and a group of Battle Sisters entered the keep, while the Stormtroopers launched a diversionary attack. They fought their way through the traitor PDF that made a majority of the cult and reached the grav lifts that would take them to the chapel. Here Reinhart told the truth to Savaul and the Sisters and told them the Black Templars would not join them in stopping the ritual. They had a mission to make their way to the catacombs, where they were to install a wounded Sword Brethren into a Dreadnought to save his life. Shocked and angered Savaul and Helena argued with Reinhart, but the Black Templar stood firm. Though the odds were against them, Savaul, Helena and her Sisters were still determined to stop the cult and acceded to the chapel. They fought their way to the entrance to the chapel, but faltered against the cult's superior numbers. Just then, Reinhart and another Black Templar appeared and destroyed the cult members guarding the entrance. Swayed by her words, Reinhart had sent the remainder of his group to complete their mission, while he helped Helena and Savaul.
Inside the chapel, the heretic priests were close to opening the warp gate. Unable to attack the priests because of a physic shield, Reinhart entered the shield and detonated a cluster of grenades in his arms; as Helena and the rest of the group held the surging cult back near the entrance. The resulting explosion succeeded in stopping the ritual, though Helena was killed in the blast.