Doomwake

A spectrum of Giants claims territory across Theros, drawing Strength from aspects of the world itself—from ancient stone and roiling flames to the depths of the seas and skies. Unlike many creatures of legend, most Giants owe their existence not to Nyx and the dreams of mortals, but to natural forces in the land. As a result, Theran Giants are typically infused with the elements they embody, which might manifest as flaming beards, frozen skin, hair crackling with lightning, or other primal incarnations. Beyond these primal Giants, though, other Giants fill the tales of Theros.
Belief is a powerful force in Theros, capable of bringing about wonders and changing the world—but not always for the better. Doomwake Giants arise from pernicious fears that infect a whole city or region. If enough mortals dread destruction for long enough, their terror can manifest as one of these massive Nyxborn beings. Once unleashed, a doomwake giant goes on a rampage, fulfilling the prophecy mortals believed into being. After doing so, these terrors typically roam to the edges of the world, where they either Fade Away over time or become legends that take on their own terrible life.