Reborn as a Primarch, My System is Too Serious!

The galaxy screams. Macragge, humanity's last bastion, crumbles under a tide of screaming demons and twisted traitors. Even the legendary Ultramarines are broken, their greatest heroes overwhelmed, despair a tangible force as the Emperor's light fades. Just as extinction looms, a soul from another world awakens in a legendary body: Roboute Guilliman, the mighty Primarch. He's furious, he's powerful, and he's not alone. With the Emperor's Sword in hand, he carves a path through the carnage, reclaiming lost glory. But his true 'golden finger' isn't what he expected—a 'serious' System promises ultimate tech and power from belief, but can even a god-tier demigod turn the tide when the galaxy itself is already tearing apart?