Walkers

Somewhere between worlds, there is the truth.

— Walker Manifesto

Walkers are magicians, warlocks, witches, or otherwise capable humans who have unlocked the ability to walk between worlds. More often than not, this means between the Shadow and the Prime, but some mad bastards venture well into the Beyond. There's little official business about it - nearly everyone in the Shadow can Walk, or they wouldn't be there.
For the most part, to Walk requires three things - magical talent, a Gate, and a Key. If you have all three, walking between worlds is a simple, if dangerous, matter.

Ways of the Walkers

Safety, secrets, sanctuary... There's any number of reasons to Walk.

Most Walkers are some form of magic-user, save for the rare few whose arcane ability is something raw and natural. For them, walking the worlds is a matter of fact - a facet of daily life. The further along the path of a walker someone goes, the more bonds of the Prime they eventually give up. The Shadow, while not a pleasant place, is to many a higher place of being. Here, magic flows free and ancient, arcane feuds are indulged. Experiments, spells, and rituals all work so easy here.
On the other hand, just as many Walkers feel the pull of their real lives keenly. To them, magic is something they can use to reinforce their place in the real world, not replace it. The Shadow is a place to escape to, to plan from, or to recharge.

To monsters, the matter of Walking is both less and more complicated. Only some have the ability to travel as Walkers do, such as Vampires or Time-Eaters, while others can only do so when conditions are right.
As the Shadow bleeds into the world and a deep mist descends upon a lonely forest, the howls on the wind come from across worlds.

There's not much of a formal structure around Walkers. New spellcasters or awakened eventually find their way to the Shadow, pulled by forces of destiny stronger than any chains, and from there they tend to pick up the rest through either charity or indentured servitude. Other make bargains with things from the Beyond to unlock these secrets, trading freedom for power.

Tools of the Trade

Besides Magic and with everything else in order, Walkers require a Gate and a Key to be able to travel. Training comes quick, though finding the necessary Keys can require barter or force - many are in the hands of things that have no interest in whatever some Walker desires to do. Not all Gates are easy to reach, either; it is said there is a Gate to the Beyond somewhere in the Marianna Trench.

Gates

Keys

Travel Restrictions

Sometimes, Walking doesn't work the way it should. Magic is unpredictable and the winding ways between worlds follow their own inscrutible whim. Journeys that should take minutes can suddenly take days, or the reverse. Reliable pathways are highly prized and jealously guarded secrets.

In places where the ward of worlds is weak, travel becomes easy. Some places are so filled with misery and death that dangerous things cross over almost by accident.

by http://www.tribalartasia.com/

To Infinity, And Beyond

A few Walkers specialize in heading into the Beyond; a dangerous realm that lie past the Shadow, where gods and demons live. Going there follows many of the same rules as travelling between the Shadow and Prime, but the stakes are so much higher.

Travel Pass

Some Walkers make deals with things beyond mortal ken for their Walking, trading anything from service to souls for their aid in Walking. On the plus side, Walkers so imbued can usually take short-cuts not available to others, such as creating keys to certain Gates on the fly. Mundies, mortals with no magic training, sometimes travel through these bargains.
Making such deals is not for the faint of heart, and it is never sure what exactly the alien minds of the Beyond want, or when they will collect.