Heldalf, known as the Lord of Calamity, is the primary antagonist in Tales of Zestiria the X. Once a highly respected general in the Rolance army named Georg Heldalf, he was entrusted by the previous Shepherd, Michael, with protecting the village of Camlann. Despite this trust, Heldalf betrayed Michael by using the village as bait to ambush Hyland soldiers, turning the area into a war zone and leading to its destruction. In his dying moments, a vengeful Michael invoked a powerful curse on Heldalf, sacrificing his own nephew to condemn the general to a life of eternal solitude and immortality. This curse prevented Heldalf from dying, causing his family to perish and his life to crumble as he became a reviled figure. Driven to despair and unable to end his own suffering, he eventually succumbed to malevolence, transforming into the most powerful hellion known as the Lord of Calamity. In this form, his appearance is monstrous, resembling an anthropomorphic lion with dark skin, a brown mane streaked with darker shades, purple claws, and a chain-like tail ending in a dark mist. He wears an ominous black coat that reveals an otherworldly purple mist at his core.

Driven by his curse and despair, Heldalf seeks to spread malevolence across the world by instigating war between the kingdoms of Hyland and Rolance. His personality is defined by a deep-seated bitterness and a desire to turn the Shepherd, Sorey, into a hellion, often employing underhanded tactics to mentally break him. However, his motivations are rooted in a tragic desire to end all sorrow and suffering by destroying all humans and seraphim, believing it to be a form of liberation. Despite his villainous facade, a fragment of his original humanity remains, compelling him to unconsciously battle the malevolence inside himself. This is most evident when he retreats to the Northern Lands, hoping to die alone and far from others to lessen his impact on the world.

Within the story, Heldalf serves as the direct foil to Sorey. He views the young Shepherd as a source of both intrigue and irritation, wanting to corrupt Sorey's pure heart which cannot be consumed by malevolence. He is aided in his schemes by Symonne, a seraph of illusions who serves under him. Throughout the narrative, he appears at key locations like Glaivend Basin and Pendrago, engulfing entire areas in malevolence. In the climax, he fuses with the seraph Maotelus to become a dragon-like hellion of immense power. Unlike the original game's conclusion, the anime sees Sorey resolve to save Heldalf rather than simply kill him. Recognizing that Heldalf still fights against his curse, Sorey opens a passage into the earth's pulse and seals himself alongside the Lord of Calamity. An unknown amount of time later, Sorey successfully purifies Heldalf, restoring him to his human form and freeing him from both the malevolence and the curse. Heldalf is then seen beginning a new, peaceful life in a faraway town.

In combat, Heldalf is a formidable opponent who fights open-handed and commands various elemental attacks. He possesses the arte "Lion's Howl," a secret technique from his days as captain of the Platinum Knights. Other notable abilities include using Karmic Stance as a counterattack, summoning hellions, and absorbing souls, as demonstrated when he absorbs the seraph Zenrus. When armatized with Maotelus, his power increases to a planetary scale, allowing him to corrupt the entire world with malevolence. His most devastating seraphic arte is an altered version of "Indignation," which unleashes a maelstrom of lightning. Despite his immense power, his immortality is tied to his bond with Maotelus.