Official Canon Edit
Before Guilliman's internment in stasis, he comissioned Cawl to work on a special project for him to create the next generation of space marines, giving him unprecedented access to the original primarch cloning labs and the custodian gene seed. Cawl spent the next 10,000 years to create not just the Primaris Marines themselves, but also improved versions of all their armaments. The project was ready to be unvieled by the time Abaddon the Despoiler attacked Cadia in his thirteenth Black Crusade.
Cawl aided in Cadia's defense but the planet was ultimately lost. However, with the help of the Ynnari, the Imperial forces were able to escape to Macragge. Cawl had prepared a special new suit of armor for Guilliman, but needed Yvraine's help to revive him. With Guilliman revived, Cawl was able to unveil the Primaris Marines. However, as time passed, Guilliman became concerned with Cawl's sanity as he made more forceful demands, including making Primaris Marines of the traitor and lost legions.
TTS Canon Edit
In a letter, the Emperor was told that Cawl was planning "something awesome" akin to the Thunder Warriors. The Emperor dismissed the idea, believing that the orthodoxies of the Mechanicus would prevent anything from coming out of it. However, Rogal Dorn told him that Guilliman had mentioned Cawl once, though he couldn't remember the context.