Via our good friends at Lexicanum
While fighting as part of the relief force for Hades Hive during the Second War for Armageddon campaign, Calistarius became a victim of the Red Thirst. After being inducted into the Death Company, he took part in the assault on an Ecclesiarchy building and was one of many trapped inside when the building collapsed during battle. For seven days, Calistarius lay trapped in the rubble, teetering on the edge of death and madness. Somehow, rather than succumbing to the Red Thirst, he managed to conquer it. By sheer strength of will he was able to suppress and hold in check the feelings of rage and the desire for blood, and in doing so he became something more. On the seventh night he burst free of his rocky tomb, reborn as Mephiston, the Lord of Death.
Mephiston’s resurrection did not go unwitnessed. By this time Hades lay once more in the hands of the Imperium, but Orks still roamed the ruins. As Mephiston heaved debris aside from his tomb, the sound of tortured stone drew the attention of one band of Orks. Weaponless, and with his armour shredded and mangled, Mephiston must have seemed easy prey, but nothing could have been further from the truth.
His gene-seed, dormant these many long years, had awakened and wrought further changes, granting exceptional strength and vigor. Moving with a speed the Orks could not match, Mephiston unleashed a flurry of attacks, every blow pulverizing flesh and shattering bone. Five Orks died in as many seconds, and a dozen more swiftly followed. The greenskins never stood a chance, but they were as stubborn as Mephiston was determined. It was not until the reborn Angel punched clean through the biggest Ork’s chest and tore out his heart that the survivors fled. His ruined armour slick with the blood of his foes, Mephiston began the long walk to Imperial lines.