"The Omnissiah filled the galaxy with mysteries so that we might learn from them, coming step by step closer to his perfect being. To ignore them, even in the face of war, is heresy."
- Archmagos Belisarius Cawl
"His colleagues are limited. Their beliefs have become a faith that they dare not challenge. The Adeptus Mechanicus is far more trammelled in its thinking than the Mechanicum of your time was, my Lord Guilliman, and the archmagos was a radical in those distant centuries. You would not have come to him if he were not."
- Cawl Inferior, Cawl's not-an-AI
"I suppose I could part with one and still be feared..."
- Cawl, on his various Doomsday Devices
Archmagos Dominus Belisarius Cawl is the first unique character for the Adeptus Mechanicus faction, and like all good servants of the dragon on mars has turned himself into a FUCKING CYBORG. Along with Inquisitor Greyfax and Saint Celestine, he forms the Triumvirate of the Imperium. This motherfucker is old. How old? Ten thousand years, which should be impossible as the oldest the mechanicum can get is around 1000 years and they also go insane, but who knows. He might have spent most of it in stasis, or built himself new bodies and implanted his not-an-AI into it, but the most popular rumors among the Mechanics have it that he did so using some kind of forbidden xenos tech while replacing what few organic parts he had left with machinery, Ship of Theseus style.
He has some sort of mysterious mission he's been on for the last ten thousand years. It was given to him by /tg/'s favourite person, apart from a certain Spiritual Liege and consists of two parts. The first is making Guilliman a brand new suit, making Belisarius Cawl the slowest tailor in all of fiction (though admittedly that's because it went on hold until he could figure out the 'resurrection' thing). The second is to build brand new weapons and pull out old ones to help fight against the enemies of the Imperium. his main creation was the Primaris Marines which took him nearly 10,000 years create (although this is because how ridiculously advanced the Emperor's work was so hard to decipher (and even then he may have just ripped off part of the Adeptus Custodes creation process.))
He also sounds like a Dalek.
Fall of Cadia
Cawl is introduced at a technoarchaeological dig on the planet Eriad IV, a planet only notable for having a small Imperial outpost that was destroyed in a Black Crusade. Although his progress is interrupted by an Orkish invasion, he continues onward, following the mysterious hints of the Shadowseer Sylandri Veilwalker. Eventually, he digs deep enough to find the remnants of ancient obsidian pylons. Cross-referencing this with similar discoveries, he realizes that Abaddon has made a point of destroying similar such pylons across his Black Crusades. He puts two and two together, and immediately starts heading for Cadia, fearing that they might be the last pylons left in the galaxy.
After arriving at Cadia and briefing the command about his discoveries, he went about studying the pylons, believing that they held the key to Abaddon's defeat. This was a dead end until Trazyn showed up and gave him some Necrontyr tech-support. With his help, he managed to activate the true power of pylons, shrinking the Eye of Terror and fucking up all warp activity on Cadia. This was a double-edged sword, as while it prevented Daemons from manifesting, it also forced the Legion of the Damned out of the Materium and greatly weakened Saint Celestine. Despite this, the defenders successfully got the Black Legion to retreat, and victory looked inevitable.
Then Abaddon just had to crash the remnants of a Blackstone Fortress into Cadia. Cawl's efforts were for naught, as the pylons were destroyed and the Eye of Terror expanded to consume the Cadian Gate. He retreated on his Ark Mechanicus, the Iron Revenant, but was pursued by Abaddon, who just learned of a mysterious relic Cawl had and sought to claim it for himself. Cawl was forced to sacrifice the Iron Revenant to stave off the Vengeful Spirit and took his artifact along with the rest of the Imperial forces to the ice moon of Klasius. Cornered by the Black Legion, all seemed lost, until they were saved by the timely intervention of the Ynnari Eldar, who offered an escape route through the webway and an alliance.
Rise of the Primarch
This is where it's revealed that the artifact Belisarius has is the Armor of Fate and a self-fitting dressing room, commissioned by Grandpa Smurf several millennia in advance. Cawl, alongside the Ynnari, successfully uses this to resurrect Guilliman and then assists him in his crusade to Terra. When they arrive Guilliman tells Cawl to work on the second part of his mission, that of strengthening Imperial forces by pulling out all the stops on Mars.
It has since been revealed that this "second part" was the creation of the Primaris Marines, along with their new wargear.
Uncle Cawl for Fabricator General!
Cawl has pretty much been the only Magos that has had the desire to innovate (and survive to do so) in the last 10,000 years. A Magpie Magos, Cawl has no qualms about claiming that human technology and understanding is lacking in ways compared to the older races. Which makes him more than willing to incorporate any xeno technology, slap the emblem of Mars on it, and let it benefit mankind. As a result the list of "Tech Heresy" he has committed is only marginally shorter than the list of awesome new stuff he has built. Even more radically, he has gone on record claiming that the loss of the spirit of inquiry was an infinitely worse loss than any of the STCs and that as a result none of his "brothers" can create anything truly new.
Currently wants Guilliman to appoint him Fabricator General of Mars so he can "Get Shit Done", but Guilliman is unwilling to have all of the tech priests lose their minds over it; given his outspoken views on the value of innovation and thinly veiled contempt for the accepted AdMech dogma such a promotion would be certain to result in a civil war within the Mechanicus.
Cawl also has experimented with creating Primaris Marines of all the Traitor Legions, as well as the infamous missing Legions II and XI. He petitioned Guilliman to sanction their use, but you can guess how that went. (Guilliman has a feeling that he's just going to try and make them anyway). He is also trying to make new pylons to shrink back the Great Rift; perhaps it's time for Cawl to pay a visit to Illuminor Szeras, or maybe politely request (at gunpoint) Magos-Explorator Omnid Torquora to tell him all the knowledge he has left of that empyrical bomb he used in the novel Adeptus Mechanicus: Tech-Priest.
Oh, and that AI we talked about before? It's not even the only one. Guy Haley's In the Grim Darkness of the Far Future openly mentions that Cawl's mind is essentially composed of numerous AI copies of himself operating in tandem with his original mind, in what he compares to being a conductor of an orchestra with the AI copies as the musicians. Which in effect makes him practically immortal, since he could switch on a backup copy of himself if his original body dies.
On the Tabletop
Belisarius Cawl is quite a formidable opponent. His ranged weapon of choice is the melta-gun on steroids Solar Atomizer, and for melee he's got a Mechadendrite Hive to fight hordes, Arc Scourge to fight vehicles, and a power axe to fight terminators. He also brings across the three Canticles of the Archmagos, which are like the Canticles of the Omnissiah but they also affect friendly vehicles not part of the Cult Mechanicus faction. Harmony of Metallurgy gives It Will Not Die, Utterance of Neutralization increases Ballistic Skill, and the War Hymnal of Fortitude grants invulnerable saves.
Not only can he dish out the pain, he can take it as well, with a 2+ armor save, Toughness 6, Feel No Pain (which is rerollable if he's the Warlord), and five wounds which he restores 1d3 of each round. Oh, and he can also use his canticles to give himself three It Will Not Die rolls. However, he lacks Eternal Warrior, and his Invulnerable Save is a mere 5+, making him vulnerable to Instant Death.
Despite all his cheese, he'd probably still get beaten by Anacharis "I fuck Primarchs for breakfast" Scoria.
M6" WS2+ BS2+ S5 T6 W8 A4 Ld9 2+/5++, a special rule that allows you to add or subtract 1 when rolling on the Canticles of the Omnissiah, allows friends Mars units within 6" to re-roll misses, and heals D3 wounds each turn. And his Solar Atomiser is an Assault D3 S10 AP-4 unholy beast that does D3 damage, unless the target is within half range, which then it does D6 damage.