Belisarius Cawl

Archmagos Belisarius Cawl is a tech-priest of the Adeptus Mechanicus. He is a close ally of Roboute Guilliman and is the chief architect behind the Primaris Marine project.

Official Canon
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 purest primarch 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
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.

Per Eldrad Ulthran’s instructions, Cawl was to be recruited by Cypher and Sylandri Veilwalker should his plan to summon Ynnead failed.

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 intercepted Kitten and his Custodian force on arriving at the Noctis Labyrinth, disabling their grav-carrier. Kitten's shield-host would proceed to engage Cawl's large force of Skitarii Rangers and Kastellan Robots, the latter succeeding in killing several Custodians, as well as Apollo Diomedes, the only one among Kitten's forces to score a hit on Cawl himself. Cawl seemed to take being hit in a mix of anger and hysteria. When The Lockwarden and his Shadowkeepers and Santodes joined the battle via Deep Strike, successfully eliminating multiple Kastellan Robots, Cawl ordered his Datasmith to enact the Chant of the Machine Spirit's Vengeance. This caused the downed Kastellan Robots to explode right under Kitten, being impaled by a disembodied arm of one of the robots, ultimately killing the Captain-General.

Trivia

 * Cawl's way of speaking with multiple voices is reminiscent of The Master from Fallout 1.