During the extensive fortification of the Imperial Palace, in preparation for the Traitor Legions' inevitable attack, Katafalque was ordered by Rogal Dorn to command a sub-orbital platform ferrying workers and construction supplies to the building site.
This unfortunately brought him into conflict with the Legio Custodes, who had not been informed of the arrival and considered the platform's presence a threat to the Emperor's safety.
Neither side would back down, and the Custodians conducted a hostile boarding action of the platform, but Katafalque gave his men strict orders not to open fire on the Custodes.
Eventually, an intervention by the Sisters of Silence allowed the incident to be dismissed as a simple misunderstanding, but both the Imperial Fists and the Custodes saw it as an ominous sign of the kind of confusion that could give the Traitors an opening in a crucial moment.
At the Siege of Terra, it was Katafalque who bled with his men before the walls of the Imperial Palace, under the horrific onslaught of the Warmaster Horus' siege, putting his body between the enemy and his Emperor. He made them pay in blood for every treasonous step.
Following the end of the wars of the Scouring, the Imperium's military forces underwent a drastic reorganisation, which included the Space Marine Legions. With the adoption of the Codex Astartes, the Legions were to be broken down into thousand-warrior Chapters.
Though Rogal Dorn at first opposed breaking up his Legion, he later axquiesced when it became clear that his refusal might lead to a second Imperial civil war so soon after the first.
During the Second Founding, Rogal Dorn designated Demetrius to become the first Excoriator; the first Chapter Master of a Successor Chapter of the Imperial Fists, as a reward for his valour.
Katafalque later scribed the sacred tome The Architecture of Agony, a treatise dedicated to devotional suffering.
Demetrius favoured their Primarch's practice of the self-infliction of pain for his brother Excoriators. Such rituals had been favoured by Dorn personally as a form of martial communion and as purification for the soul.