Official Canon Edit
In the early days of the Imperium, Cawl was a simple if unorthodox tech-priest. He was extremely ambitious, having enhanced his brain and ended up absorbing the memories of Ezekyle Sedaine, the inventor of the Black Carapace, after Sedaine attempted to take over Cawl's body. 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 unveiled by the time Abaddon the Despoiler attacked Cadia in his thirteenth Black Crusade.
Note: the following canon will be changed per Cawl's inclusion into TTS.
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.
Cawl was summoned by the Fabricator-General of Mars to eliminate the Custodian shield-host before they reached the Noctis Labyrinth and took the Proteus Protocol. Cawl was eager at the chance to steal some Custodes body parts to add to his Primaris Marine project. He seems to have absorbed several personalities at this point, causing him to speak and act erratically, including sometimes shouting in homicidal rage in between responses.
- Cawl's way of speaking with multiple voices is reminiscent of The Master from Fallout 1.