I have run two rpg-groups in the Masque of the Red Death-campaign for well over a decade. One is situated in England and the other one in Germany in the 1880s.
An important part of the MRD-campaign world, which is more or less identical to our Earth in victorian times, is the existence of a mystical, dark force. Sometimes referred to as the Red Death. This force entered Earth in the 27th century BC when the Egyptian architect Imhotep summoned it by accident while building the pyramid of the pharao Djoser. It is described like this:
The Red Death is a mysterious evil energy that can manipulate the world. Its very origins are unknown to anyone, but many theories are abound. Some say it is very much like a physical red mist, while others claim it to be a universal force like gravity.
What is known is the effects that the Red Death has. It is capable of raising undead, transforming people into monsters, driving people insane, moving whole areas and locations, and affecting the very fabric of reality. The Red Death is an extremely powerful force that comes and goes at its own whim, and is the source of all evil in the Masque of The Red Death world.
There is given no explanations in the campaign material as to where it came from, or even what it is, but it slowly extended its corruptive influence over the world, spreading from its starting point in Egypt.
A major power of the Red Death needs special mentioning. It can keep planar creatures captive on Earth. Creatures can be summoned or travel to Earth on their own accord, but they may never leave again (for creatures in desperate need of sanctuary that could actually be an advantage, since no powerful creatures would follow them there, at least if they were aware of this feature of the planet).
The idea is probably that the fight against this dark force is meant to go on indefinitely, so there is no real need to go into details. But I decided long ago that my players were going to have an actual opportunity to end this menace and liberate the Earth from its power.
I intend to keep that silent promise, especially after I started my Planescape-campaign a few years ago, and a small group of qabalists ended up leaving Gothic Earth (something that should not have been possible, of course). I more than suggested to these players that they might finally find the answers on the Outer Planes that their organization had spent centuries searching for on Earth, without success.
I have a few ideas already of course. Maybe it is some sort of Yugoloth experiment that has escaped, or maybe Gothic Earth itself is an experiment. Perhaps it is a Baatezu weapon gone astray. Maybe it is some sort of deity. The power to keep planar creatures captive might suggest some connection with Carceri.
I´m still a long way from coming up with a satisfying explanation though.