The Sisypheum was an Iron Hands Strike Cruiser that played a crucial role at the outbreak of the Horus Heresy, evacuating loyalist Legionnaires from the surface of Istvaan V before escaping and beginning a series of Raids and strike operations against the Traitor Legions and their Primarchs.
The Sisypheum was a highly modified Strike Cruiser, compact and deadly, the vessel was Up-armoured and with a black, non-reflective hull that had been repaired and upgraded by it's Crew and the expertise of the Iron Hands Legionnaires themselves. It contains a laboratorium in the upper levels where Iron Father Frater Thamatica, Ironwrought of the Morlock veterans, conducted dangerous High Energy experiments with advanced technology, in order to try and give the Loyalists an edge.
The Vessel was Commanded by Captain Ulrach Branthan of the 65th Clan Company, Command passed to his Equerry Cadmus Tyro as Ulrach was mortally wounded and held in a Stasis field in the Apothacerion.
Istvaan V and Subsequent actions
The Sisypheum escaped the betrayal at Isstvan V, but was crucially able to evacuate a number of Personnel off Planet recovering a large Contingent of Morlocks and Iron Hands including a Raven Guard Legionnaire Nykona Sharrowkyn and Atesh Tarsa an Apothecary of the Salamanders Legion.
Death Guard, Sons of Horus, Word Bearers and Iron Warriors had all taken their best shot at destroying the Sisypheum at Isstvan V, but it had eluded them all enough to escape when the net was closing in. A Night Lords vessel had come closest to ending its defiance, but employing subtle tactics no one would have expected from an Iron Hands captain, the Sisypheum managed to out manoeuvre VIII Legion’s Tenebraxis. It raked its rear quarters with fire, leaving it wallowing and defenceless, the Tenebraxis was then boarded by kill teams of Iron Hands from the Sisypheum who took the fight to the enemy in their shadowed halls and darkened companionways, stripping it of anything potentially useful for refitting and repair work. Leaving the stricken ship burning in the ice floes of the Isstvan cometary belt, the Sisypheum fled the system. Using supplies and equipment liberated from the Tenebraxis the Ship's Company was able to repair most of the battle damage and ensure they had adequate supplies.
It escaped the clutches of the Traitor Legions and began a hit and run campaign against the rear area's of Traitor Space, intercepting Convoy's and picking off individual Naval ships and Traitor Legion ships. It began a campaign of Intelligence gathering and surgical strikes and managed through the exploits of two members of it's Compliment Nykona Sharrowkyn and Iron Father Sabik Wayland.
Using the Sisypheum as a base of Operations Sabik and Nykona were sent to secure and extract a Kryptos, they were successful and thereby the loyalists began to decode Traitor Communications. 
Once again the pair were inserted on to Hydra Cordatus by the ships smaller craft, to intercept the meeting of the Traitor Primarchs, and after a failed assassination attempt on Fulgrim they returned to the Sisypheum. The decision was then made to intercede in the Traitors plans, taking the Ship to the dead Eldar world of Iydris.
En route to Iydris, One of Frater Thamatica's experiments went dreadfully wrong and compromised the vessel, exposing the Ship on the Sensors of the entire Fleet of the both the Iron Warriors and Emperor's Children Fleets. The two Primarchs argued over the course of action and the III Legion boarded the vessel, not to be out done by the his brother Perturabo teleported directly across with the members of his Bodyguard and his Command trident. The Ship was dangerously unstable and only a last minute intervention by Thamatica prevented the ship Core from exploding. Sensing something was wrong Perturabo evacuated and the Emperors Children Followed, and the Sisypheum suddenly due to Thamatica's actions underwent an incredible acceleration and cut directly through an Emperor's Children vessel. Destroying it utterly and managing to escape from the traitor legions.
Later, the Sisypheum under Cadmus Tyro allied with Shadrak Meduson in raiding an Alpha Legion outpost. However when it was revealed that Meduson had in fact been Alpharius in disguise and the entire battle was a manipulation, Tyro and his forces fled.
- 1: Kryptos (Short Story) by Graham McNeill
- 2: Angel Exterminatus (Novel), by Graham McNeill [Needs Citation]
- 3: The Seventh Serpent (Novella)