Mortarion didn't keep the secret to himself. Once it was out, he generously shared the formation with all his brothers.
While Magnus was still there, Little Pony took charge of copying the formation. He joined forces with Mortarion and Lorgar, as well as Khan, who had some knowledge of psychic powers, and made one for every Primarch.
As for Hogg, he kindly provided the Adamantium base that would serve as the main body, along with the precious materials used to inscribe the formation.
But Hogg knew nothing about psychic powers, so he didn't meddle. Instead, he got creative with the formation's base. He took a Baneblade heavy tank to use as a mobile chassis, then, in proper Ork fashion, crammed two enormous plasma engines into it for power.
During the modifications, however, Hogg realized there wasn't enough room. So he kept adding material, then adding water, until he finally put together a colossal mobile fortress, thirteen meters tall and comparable in size to a Knight Mech.
Once they had a template, the rest was easy. With C'tan Power, Hogg didn't even need to lift a finger; he could assemble the parts with a mere thought.
He even prepared one for his second brother, Roland, who hadn't come along. As a thoughtful touch, he painted it macho pink to give him when they met again.
Everyone knew about the formation beneath Terra. Fulgrim had even drawn from it for several hundred years, so no one questioned it. They treated it like a lottery machine: if you got something good, it was luck; if you didn't, no one minded. In short, they left it all to fate.
To ensure the attempt succeeded, every Primarch used a neural link to experience the peculiar sensation through Mortarion's projection and prepare themselves.
But just as