Ferrus Manus

Ferrus Manus ruled Medusa's clans and led the Iron Hands, his arms sheathed in living metal after a duel with the wyrm Asirnoth. He wanted his Legion to be as unyielding as the metal, and they learned that lesson too well.

Fulgrim killed him at Isstvan V, the first primarch to die in the Heresy. His head became a trophy the Emperor's Children paraded and Horus later kept like a counsellor who could not answer back.

The Iron Hands have never forgiven weakness since. They still speak of the day the Gorgon returns. Medusa does not. It only works. Medusa's clans still send their sons to a Legion that treats grief as an engineering problem.