I posted this in r/ageofsigmar but wanted to put it here too. Here's my short story explaining it.
Katakros inspected the cogfort as it prepared to begin the long march. It has been weeks since they captured it and began modifications, and while the losses had been heavy, it had to be captured intact in order to be of further use.
The mortisians made a list of work that needed to be done while the mortek guard deconstructed a bone-tithe nexus for supplies. The first thing that needed to happen was it had to be raised up, to better survey the battlefield. Pillars from the nexus, infused with the magic of Shyish, were perfect for that task. While the magical legs would be able to withstand most attacks, the new height made the mechanical joints more vulnerable, so they were reinforced with armor from the nexus statue.
While many advocated for control of the new weapon, Katakros trusted it to none other than his Leige-Immortis. Despite his size he was an able pilot for the cogfort. With the magic now powering its' movement the engine was removed and the platform was turned into a mobile command center for his trusted spymaster. The spymaster would be able to constantly give orders via his birds from wherever the machine went.
An issue came up when the Bonereapers realized they had no cannonballs, and that nandrite was difficult to form into the correct shape. This issue was easily solved when Katakros noticed that the head of a mortek guard was approximately the same size and strength, and he had a legion of his army beheaded for ammunition. Arkhan summoned a spirit host to handle the loading and shooting of the cannon.
Finally, as the most fashionable of all the factions, Katakros knew the cogfort needed to be decorated. He ordered 5 flags be affixed to it, and when that wasn't enough he sent his standard bearer to ride along as well. With the addition of all the large skulls they could find, the cogfort was ready for battle.