Nullstone

The Nullstone is an artifact of the Church of Tyranix in Zardis, used to create the anti-magic field which covers the nation and is slowly expanding across neighbouring realms. It was created by stone carvers and priests of Tyranix working in a divinely-inspired trance, who gave their lives to it at the completion of their task.
The stone is a large black basalt altar, ten feet long by five feet wide and high, carved with runes that glimmer with sullen red light and hurt the eye to look at. Spiked black iron chains hang from the corners and middle of each side, and animate when a spellcaster is dragged toward it, writhing eagerly.
It is the task of the Links of Tyranix to keep the Nullstone fed, and they carry out their job with alacrity. Whenever a spellcaster is found, whether within the borders of Zardis or nearby, they are captured, imprisoned in magic-dampening manacles, and dragged to the altar. The chains wrap around the altar's victim, the spikes on each link digging in and drawing blood, before hauling the individual onto the altartop and starting to drain them of magic and life. Eventually, when they are near death, a priest of Tyranix steps forward and raises a ceremonial knife high. As they cut the victim's heart from their body the altar drinks the blood, runes flaring with sickly red light that does not illuminate anything in the surrounds.
The divinely-powered anti-magic field created by the Nullstone does not block priests or warlocks of Tyranix from casting their spells, only arcane spellcasters and clerics of other faiths. They find themselves cut off from their thaumaturgical strength as soon as they enter the area of effect. The field covers the entirety of Zardis, and expands gradually every year as more and more magically-gifted individuals are fed to its horrid purpose.
The mages of the nearby island Azoria have only the faintest idea of its existence and purpose, and have repeatedly tried to send agents into Zardis to obtain more information. Every one of them has been exposed by the Links of Tyranix and fed to the Nullstone.