Religious event
As societies and cultures flourish beneath their watchful eye a baleful prophecy warns of the destruction of Gyrraholm, ushered in by the Monumentals themselves. This warning can mean only one thing, the Monumentals must leave the world.
Decades after the construction of their Kingdoms on Gyrraholm, and the burgeoning civilizations that they had nurtured had grown to cover much of the world, the Monumentals grew cautious of their kin. What would stop one Pantheon from turning their power on another, and claiming more of Gyrraholm for themselves? As the paranoia took hold, the more magically inclined among them would receive horrible visions, telling of the End of the Monumentals, and the destruction of the world itself. As with even the most powerful divination magics, it's impossible to predict the future perfectly, and these visions are vague. They foretell of the world ripping apart at the seams, as oceans rise to engulf cities, while storms ravage the lands. But what was clear was the presence of Monumentals, and their seemingly prominent role in the breaking of the world. Their love for their creations and their fear of their destruction outweighing any potential paranoia among themselves, the Monumentals agree that it's best to leave Gyrraholm in the hands of mortals, and retire to Demi-Planes formed from the faith of their followers. The Valorians retired to the golden summit of Parádeisos. The Vikir to the shining mead halls and fields of constant battle found in Vahǫll. The Shams'anti make their way to the celestial oasis known as Aaru. The Deiwild retreat to the endless cosmic woodland of Fiáinspéir. The Mana Atua travel to the island paradise of Te Rangi. The Okami reluctantly relocate to Tengosai, the cosmic citadel. The Dwarven Bozha retire to the glittering caverns of Mount Reij. The Halfling Children of Yondalla the green fields of Venya, though much like their mortal kin find themselves welcome among most (if not all) of the Monumental Demi-Planes. And thus it would be that no Monumental avatar would set foot on Gyrraholm, an agreement that would only be broken during the world's darkest hour, when the prophecy would reach it's climax, leaving the Monumetals with casualties, and the face of the world forever changed.