Here is one for Snowbluff. I based it around the logical conclusion of the Snowbluff Axiom.

The Living Plane

Eons ago a mage of terrible power sought to breach the wall between magic and reality. The mage believed that the prime material could be made more malleable by bringing it into alignment with the plane of Limbo, and using that malleability would tear the veil between magic and mundanity.

The mage was successful at this attempt, using magics both great and strange a shard of the Slaadi's Spawning Stone into the Prime Material. The mage was not present at this event, but far away in a personal demiplane and working through an army of simulacrums of the mage. As the stone was so placed the backlash across the cosmos was terrible, closing the borders of many planes of existence. The mage was bound to his/her demiplane, stuck forever in a prison of the mage's own making.

After countless years the plane slowly absorbed the life of the immortal mage, and the two merged together. The plane now physically moved through the planes, seeking to free the mage by removing the shard of Limbo still embedded on a world in the Prime Material Plane.

The Living Plane approaches the world you are currently on in search of the Shard, drifting across the outer darkness as a great cloud of sparkling light. The closer it approaches the more strange effects appear from it attempting to reach out to the Shard of Limbo stuck in your world.

Signs

Faint
Mists and clouds begin to appear at night on the planet, especially above pools of still water. The mists carry with them incorporeal undead, especially ghosts of individuals who died in the region. The mists can be shaped by some of strong will into semi-solid, none alchemical, mundane objects (DC 16 Will Save) which have a hardness of 0. These need to be concentrated on to remain solid.

Moderate
Mists now begin to appear in day time. Crops begin to wither from the lack of sunlight, and the amount of incorporeal undead increases. As well Living Spells of spells up to fourth level now appear in the mist.

Strong
The clouds are now incredibly thick, acting as a Solid Fog on all inside. They are also now too heavy to be moved by the wind, and seem to reach out with a life of their own towards any living thing. Slaadi and other outsiders native to Limbo begin to attack individuals within the fog. Additionally the fog is strong enough to be used to make any none-magical item with a DC 24 will save. These last for a number of minutes equal to the individuals will score.

Overwhelming
The Living Plane has moved into the world and is actively seeking for the Shard. The fog retains its strength, but has now gained agency. Every turn in the fog there is a 1/100 chance of it creating a Simulacrum of the individual, which then immediately attacks them. Living Spells of spells up to level 9 wander the fog, drawn to the intense magical power of the mage. Any individual can make a DC 30 will save to transform the fog as a True Creation spell, although the fog items disappear when the Living Plane is either defeated or victorious.

The Thing in Itself
The Living Plane appears as a colossal incorporeal blob. It has no hit points and ignores all attacks directed at it. Every round it vomits out either a Living Spell (applied to any arcane spell) or a ghost Mage. The ghost is a level 20 wizard with uncanny forethought and eidetic spellcaster who knows every arcane spell. There can be only one ghost Mage outside the Living Plane at a time, and in any round that the ghost is destroyed the Living Plane creates a new ghost rather than a Living Spell. Each time the blob is forced to emit a ghost Mage it loses one size category, and flees reality when it reaches Tiny size.