The Talon of Horus pg. 396

The mace was called Worldbreaker. The Emperor had made a gift of it to Horus upon the First Primarch's ascension to Warmaster. Horus Lupercal was capable of bearing it one-handed, but the immense maul was too cumbersome for any of the Legiones Astartes to wield it with any grace. A bludgeon of darkened metal, its spiked head alone was the size of a warrior's entire armoured torso.