The Adeptus Mechanicus Forge World Pylras Five is at first unbothered by the announcement of the Warp translation of a fleet of over twenty ships that can only belong to Ork. The leaders of the world simply sent their defense fleet made up of over a hundred ships to wipe the beasts from their system. It took the two fleets five days to reach meet. Only, the clash did not go according to plan. Within an hour of the start of the engagement, reports were being sent back to the forge world from the fleet were frantic and confusing.
The fleet had barely come into firing range of the Ork fleet when the largest ships in their number reported that thousands of Orks had boarded them. It made no sense. The Mechanicus ships had barely started firing and none reported that the Orks had fired boarding pods. In fact, they had not fired at all, which is even stranger. Orks are not known for caring about range or even aiming. The flagship of the Oberon Battleship, Retribution of Steel's captain, transmits a message saying his ship's machine spirit itself is under direct attack by an unknown force. That is the last message he sends.
One by one, they fall silent. Then they roar back to life opening fire on the ships not boarded. Within an hour, the Pylras Five Defense Fleet is left in ruin. The Ork fleet, now bolstered by the captured ships of the defense fleet, powers toward the forge world. One message keeps transmitting to them over and over, over every fleet channel, in a voice that sounds like the collapse of worlds: I AM COMING FOR IT ALL.
The next line of defense is the orbital defense plates that orbit Pylras Five. All of them begin to move into position and are prepared to fire when the ships come into range. Only they are meant to be used in support of a fleet. They will do damage, but they are outgunned. Then, they all go offline. The tech priest aboard the plates reports that they have completely lost power due to a virus that has completely locked out their systems. It burned through every security protocol and anyone that was hardwired in, leaving them braindead husks. Then, just like the ships before them, they report that Orks are appearing out of nowhere. Within thirty minutes, they are silent.
During all of this, the forces of the forge world are mobilizing in a panic that any servant of the Omnissiah would find blasphemous on the normal day, but today is not a normal day. Today may very well be the end of their world. Every fighting unit in the world is mobilized and stands ready to defend the planet spanning Forge City, Pylras. They fire up their void shields and aim their anti-air guns at the sky. The defenders are all looking to the sky, ready to see the flaming trails of Ork drop craft. Only they don't come from the sky. The five hundred thousand Orks of what would become known as the beginning of the WAAAGH Eternal and all of their war machines appear out of nowhere in key positions within Pylras, behind its defenders, and the slaughter begins.