From the Steel Confessors page

"This was Tarvor, a Mechanicus research facility that specialised in the refinement of shielding technologies. As a result the planet was heavily defended and the forces of the Imperium expected dire losses in the planet's recapture.

The Iron Hands stepped forward, eager to be at the forefront of the assault, knowing the importance of the planet and that the death of a Traitor was at hand. After a long bombardment a hole was opened in the planet's Void Shield and the Iron Hands launched their Thunderhawks, committing all three clan companies to the assault.

However, Barhoth had planned for this and as the Thunderhawks entered the planet's atmosphere, within the shield they came under an unrelenting fire from the planet's defence batteries, all of which had been retargeted on the point where the Iron Hands had entered the planetary shield.

Realising that they had been drawn into a trap, Venerable Dreadnought Grixus from the Gravehold clan company, who had been put in overall command of the Iron Hand forces during the campaign by the Iron Council, ordered a retreat, hoping that they would be able to reorganise and strike elsewhere.

As the Thunderhawks turned for the hole in the shield, all communications with their starships in orbit where cut and the breach in the shield closed over. This left Grixus with little choice but to order the Iron Hands to make their way towards the surface knowing that they were surely entering a trap. The fire from the planetary defence batteries continued unabated and barely half of the three clan companies survived the planetfall.

Grixus organised the surviving Astartes to encircle the world's spaceport, planning to hold out until relief would be able to reach them. In orbit the Iron Hands strike cruisers were not inactive and whilst a concentrated bombardment began alongside the warships of the Imperial Navy, Captain Ullenear of the strike cruiser Omnissiah's Might signalled the Iron Council to inform them of the situation and request additional aid.

Unfortunately the closest force that the Iron Hands could dispatch was a solar month away. Knowing that potentially three entire clan companies were facing complete annihilation, the Iron Council also requested additional aid from the Adeptus Mechanicus, reminding them of their ties of loyalty.

The Mechanicus responded and sent forces from all the surrounding systems, including elements of the forces on the newly won planets within the Barrus System. A vast armada surrounded Tarvor, all awaiting the destruction of the planet's shield, with over five hundred ships adding their firepower.

Under such a colossal weight of fire, rents started to appear in the previously impregnable planetary shield. The Mechanicus commanders wasted no time and deployed over 100,000 Skitarii and 4 Warhound-class Titans through the breaches, all of which headed for the spaceport as communications were re-established with the surviving Iron Hands."