Doha's Anchor

Doha's Anchor is a massive block of rough-hewn gray stone standing about 40 feet tall and about 10 feet tall on a side. It is always slightly cool to the touch, even in direct sunlight, and gives off an almost imperceptible feeling of calm and general well-being. It is also one of the most powerful protective objects in the world.
Doha's anchor is a piece of stone made directly by The Creator at the birth of the world, and it has remained in its current form ever since. As such, it hedges out anything that is not from the universe that Plavithen is native too. The various shackled horrors cannot approach within several (about 3) miles of the unremarkable monolith, and their supernatural abilities simply end at the border of this zone. Spells cast by them stop at the edge as if blocked by some invisible barrier, any otherworldly substances simply vanish when thrown over the line, and the beings themselves collide with an invisible force field that stops nothing else.
Notably, this also applies to mind-controlling effects. A mentally-dominated servant instantly regains their free will (and is aware of what happened to them) as soon as they cross the boundary.
Furthermore, this exclusion area is expanding, albeit very slowly. During the time that the shackled horrors slumbered, it grew from literally just affecting the pillar itself to the area that it currently affects, making this a new threat and weapon against the influence of the shackled horrors.
This has, rather predictably, made the monolith a target of the horrors as they reawaken, but thus far, the effects of the monolith itself and the fact that it sits at the center of the capital of the Stelavorn empire mean that the destruction of the monolith is more of a desire than a plan or even a goal. The horrors cannot see into the exclusion area, cannot enter it, and cannot use abilities to affect it.