The designs are innumerable, hidden in the tombs of infamous architects - sealed in lead, cast to the bottom of seas now deserts - the ugly centre of star-metal deposits - the dead basins of volcanoes.
They speak of a tower.
Mind-breakingly gigantic, both in scope and physically immensity. Koan made stone.To gaze upon it, even the ruined stubs of hundreds of failed attempts, is to be rendered as nothing before sheer mass and determination.
It demands to be built, the idea of it too large to die, but also too large to force its way into being - perhaps a cousin of the spell, lurking on the edge of reality, waiting for a opening. The portal is the structure is the idea is the behemoth, craving a physical permanence complete and resplendent, the gravestone of a world.
Entire empires would collapse under the construction project - a hundred thousand slaves could perish in the foundations, a mere speck, the very beginning. Hope a mad, royal architect does not first find the plans, and then discover a previous generations work - a solid foundation to build upon.
Incremental with the fall of empires, nestled in the rhythm of collapse, it will grow and break minds and enslave relentlessly until complete.
Somewhere in your world, the foundations were always ready, if only you had eyes to see before.