Regulus

Regulus was the Dark Mechanicum envoy Horus kept for Mars, a priest who decided the Warmaster was the better Omnissiah and opened the channels Kelbor-Hal needed for scrapcode and civil war. He hunted STCs the way other men hunt relics: as weapons, not as museum pieces.

He served as diplomat and saboteur until Kastigan Ulok killed him at the Wrought, an Iron Hands ending for a Martian who had already sold the Red Planet. Regulus died as a traitor magos, not as a world, and not under anyone else's planetary name.

Mars produced him. The Warmaster used him. The Wrought closed the file with a bolt round, which is a cleaner footnote than most Dark Mechanicum lives receive, and a poorer one than Kelbor-Hal had promised his envoys.