As the Heresy raged, he was co-commander, along with Captain Sigismund, of an Imperial task force that was sent to Mars when Horus' influence caused a civil war to erupt on its surface. Diaz and Sigismund were charged by their Primarch Rogal Dorn, with securing the two main forge-complexes on Mars that supplied the Legio Astartes weapons and equipment, which were led by Magos Kane and Lukas Chrom. To achieve this, the task force was split in half and Diaz led his forces to secure Chrom's forge-complex - unaware that the Magos had sided with Horus and was one of the leading traitors on Mars. When his task force later arrived at Chrom's forge-complex, they were attacked and Diaz was forced to secure what resources he could from the traitorous Mago's hands. Meanwhile, Mars' civil war was nearing its end and those forces loyal to Horus, were close to achieving victory. Recognizing that the task force could not stand against their enemies' superior numbers, Sigismund issued an evacuation order for all loyal Imperial forces. After hearing this, Diaz and his half of the task force retreated back to Terra, with what resources they could take from Magos Chrom with them.
Several years later into the Heresy, when the Alpha Legion began their attack on the Sol System, Diaz had been promoted to the rank of Lord Castellan. He was also given the title of Siege Master within his Legion and was given command of Terra's Fourth Sphere of defense, which oversaw the blockade of Traitor-occupied Mars. During the Solar War's final phase, Diaz commanded the largest of his Primarch Dorn's five-tiered defensive fleets, from the Battle Barge Fortress of Eternity. As Horus's forces overwhelmed the Sol System, Diaz's fleet near Mars was assailed by the Dark Mechanicum. These attacks came not only from the Traitor-held Forge World, but also from Dark Mechanicum fleets that were among Horus' forces that invaded the Sol System. The attacks eventually led Diaz to order his fleet to withdraw to Terra, before it could be destroyed. During the subsequent Siege of Terra, he agreed to take command of the Eternity Wall Spaceport's defenses, despite his Primarch Dorn judging it to be doomed. When the Spaceport's Pons Solar bridge was later swarmed by Horus' forces, Diaz was witnessed to be among the Imperials defending it. Their sheer numbers soon began to overwhelm the defenders, but Diaz's skill left him among the last Imperials to be still standing. He never once took a step backwards and when he was eventually killed, Diaz died while still cutting down the invaders.
- 1: Mechanicum (Novel), Chapter 3.03
- 2: Praetorian of Dorn (Novel), Part 1 - Chapter 4
- 3: The Solar War (Novel) - Chapter 16
- 4: Saturnine (Novel) Part 2, Chapter 3
- 5: The Horus Heresy: Legions