The Veil is a barrier that separates the world as most mortals experience it from its arcane Shadow. Theologians call it the wall between the living and the dead, the border of heaven and hell. Scientists who stumble upon it describe it in terms of membranes and dimensional boundaries. All are incomplete descriptions of the same phenomenon: the wall between the world as mortals know it and a stranger world underneath.
The Veil is not uniform. In most places it is absolute, a seamless and impassable wall without doors. But there are thin places, locations where something happened to erode it, or where the geometry of the world bends in ways that produce overlap. At these points, with the right force applied in the right manner, a crossing becomes possible. Such places are rare, their locations shift, and the knowledge of how to exploit them is closely guarded.
Crossing
For most beings, crossing the Veil is impossible. For a select few, it is merely difficult.
The most reliable method is the work of the tattoo Scribes who can etch gateway symbols into living skin, granting the bearer passage into the Shadow. This is the most common means of deliberate transit, though it is neither cheap nor without risk. The Scribes are not always forthcoming about the secondary effects their markings carry.
Striders possess another means, able to slip between worlds with a thought or gesture, but they are few and far between.
Less common and entirely unpredictable is involuntary crossing. When a mortal faces uncontrolled rage, absolute terror, true ecstasy, or desperate need, a door sometimes appears where none existed before: a gate, a window, a path between the trees that was not there a moment ago. Those who step through find themselves in the Shadow.
The Veil is far more permissive about entry than exit. Intense emotions while common in the Shadow are insufficient to return to the real world and a mortal who crosses without a Sc's mark has no innate means of return. Securing passage home requires finding a Scribe willing to help, and that help does not come free.