The Veil divides the Mortal World from the Supernatural Realms (or Otherworld) that lie beyond.
Formed from the raw essence of magic, it prevents the often deadly creatures of the Otherworld from preying on humanity. Beyond it lie the Gods, creatures often believed to only be myth or legend and the Faerie Courts.
While some supernatural creatures such as the Sidhe, can cross back and forth at whim, this talent is uncommon.
To protect magically talented individuals from the dangers found beyond the Veil, the skills necessary to pass through it are not usually taught in Institutions of Arcane Education such as the Academy. On the spectrum of power in the Arcanoverse a witch or wizard would be considered prey by those who wander beyond the Veil.
Foolish or desperate humans often treat with entities beyond the veil. Often giving up something of themselves in exchange for a boon, they believe will change their life, but more often than not costs them far more than they ever expected.
There are both times and places where the Veil dividing worlds is at its thinnest. During these times, mortals would do well to ensure that they do not pass from their world into the Other.
Throughout the world there are a number of naturally occuring gateways between the mortal and supernatural world. Those that are known by the Arcane Congress are guarded zealously. While those that are beyond their purview are used to smuggle goods between realms.
It is also believed if one was to swim to the bottom of certain lakes or pools one might reach the Otherworld. Of course, were that true, one would have to brave whatever otherworldly creatures might infest such dark places.
For those looking to pass into Faerie, it is said the ancient burial mounds or Ireland provide another gateway, but only a fool would willingly wander into the realm of the Sidhe.