Chaos at The Doorstep

Chaos at The Doorstep was one of the earliest recorded campaigns conducted by the Paladins of Thunder, a newly created Ultima Founding Space Marine Chapter comprised entirely of Primaris Space Marines against an invading force of Alpha Legion Heretic Astartes upon their Chapter homeworld of Zephyron IV.

History
Once their Fortress-Monastery, the Halls of Solace, was built and Aemilus Castor was anointed Governor of Zephyron IV, the Paladins of Thunder began the usual activities that every chapter inside the Imperium Nihilus does; sending forces to invade worlds and stop incursions from xenos races across the sector; the Paladins of Thunder did their duty without hesitation, fighting mostly Orks and Traitor Astartes. But, one day, traitor forces staged a rebellion in Zephyron III. A few days afterwards, a Chaos fleet of the Alpha Legion descended upon the system with only the 5th and 9th Companies located in the system at the moment, alongside Aemilus Castor.

Trying to stop the system from falling to disarray, Magister Medicae Castor sent the 5th Company to Zephyron III to help House Alexios and its retainers against the Traitor Knights and cultists that were reinforced by the Alpha Legion, meanwhile, he'll fortify the capital of his chapter's homeworld, as well as use the small battlegroup as means to both evacuate civilians and to send messages to other Imperial forces nearby.

With help from the 112th Zephyrean Armoured Division and the 71st Zephyrean Regiment, Castor managed to evacuate a half of the population of Zephyron IV before the first wave of Chaos forces attacked.

Once it happened, both the capital and the Halls of Solace were sieged, stopping the forces located in those places to assist one another or other places in the planet, leaving some minor settlements and spaceports defended by the PDF or, in the worst case, at the mercy of the Alpha Legion. With this maneuver, the traitor forces also managed to stop communications between both worlds.