There is a wall between realities. Some call this wall Shadow. Most of the time, Shadow keeps each reality from intruding on the others. But on occasion, this wall weakens, the barrier between worlds frays, and the things from one universe slide into another.
Such is the case in the Shadow Stalkers campaign. Here the world outside your window is on the receiving end of a passageway through Shadow. The reason for this aperture is unknown, although several theories exist. The results, however, are quite clear – an influx of fantastic creatures to this world and, with them, a sudden radical increase in the ambient magical capability. In other words, magic and monsters are now quite real.
The new arrivals only dimly recall the world from whence they came. The passage through Shadow robs them of all but the most superficial memories. They have reached this steam-powered world with whatever they carry on their backs and a few remembrances. They are collectively known as Creatures of Shadow, and the various humanoid species among them are called the Shadowkind.
Passageways through Shadow are fleeting and can appear anywhere without any warning, a rising tide that cannot be predicted. The borders of Shadow seem to permeate our own plane. There are reports of portals taking the forms of great pools of tar, abnormally dense fog, or even waves of intense darkness. When these effects recede they leave items or creatures marooned in our world. Shadow often manifests in remote, desolate locations, and these creatures enter our world confused and alone.
The particulars of a manifestation do not seem to be at all connected with the items or creatures that come through. The same type of Shadow portal might admit a wave of vermin in one manifestation, a group of monstrous humanoids in another, and a single great beast in a third. Then there are times when no creatures are left at all, but items are washed up on our reality’s shores. This could be anything from a single glass vial filled with a colorful liquid to a sword that glows pale blue in the moonlight. There are even occasional reports of whole buildings (a keep, a tower, an entire castle) appearing out of thin air.
Passage through Shadow alters Creatures of Shadow at a fundamental level as they are brought “in tune” with the mundane world. They gain a few abilities in the process, as the ability to speak local languages. But the most disconcerting aspect is that the passage through Shadow has blurred their memories.
Shadowkind refer to this phenomenon as “The Gift of Lethe”, and it results in the dimming (and outright elimination) of memories.