The entity at the heart of the Agony Stones has no name that anyone living knows. Scholars call it the Dying One. The Tytheri orcs call it Thryg-Makaal, which translates roughly to "the Bleeding Forever." Whatever it is—a creature from another plane, something older than the current age, a god that fell—it suffered a catastrophic wound sometime in the distant past and has been dying ever since.
Its death is happening on geological time. The stone formations are its petrified tissue, still technically alive, still in agony.