The Splitting of Shadow

Whatever you do, stay in the light. Why? Because you have two shadows.

— The Phoenix

If Realmwalking was safe then everyone would do it! But the truth of the matter is... it isn't. Without the proper safeguards in place such as a solid portal or a mage of a higher circle, any other form of realmwalking is dangerous.

Rifts Within and Rifts Between

While interrealm rifts have a danger of completely destroying all that pass through due to the unstable nature of its opening within the same realm, rifts between two different worlds that are unstable can flicker in and out of existencem cutting off anything that is caught between both worlds clean through in a fraction of a moment.
While creating rifts within the realm can occur in places of wild magic such as the Wastes, once a mage of a high enough circle can cast the spell whether through an arcane gateway or through personal means, the connection remains stable and there is no risk of picking up what is known to realmwalkers as a "splitting shadow".
Safety is also assured for rifts that open between worlds, in which cases higher circles of magic are used to stablize them. However, should there be a mistake within the arcane ritual or should a mage that can barely control the arcanic forces attempt such a feat, then it is an open field for the shadows to come.

When Shadows Split, A Soul Will Die

Many that have traveled between realms simply do not only lose their lives, they get stuck in between as shadows. When that happens, the only way for the shadow to escape is to latch onto an traveler that was just as unprotected as them following them into whichever realm. A being must past through wholly in order to pick up a second shadow as sticking in one's head to see what's on the other side will not pull anything through, though if the rift is unstable a beheading may be at risk.
Once latched on, the realmwalker will have two shadows which will look unnatural and seen no matter what level of light-- so long as there is some form of light. Once two shadows are seen a battle for restoration begins. As no two souls can inhabit the same same space, the body will begin to deteriote until one soul can overpower another and sadly the tides shift to the power of the assailant empowered with unnatural rift magic.
So long as light is kept on, the battle will remain but the person will grow weaker by the day. However, should darkness claim the shadows and person a true internal battle shall be waged and in the morning only one soul will remain along with one shadow.

Fighting Back Shadows

In order to rid the person of such a condition, because of its unnatural and arcanic occurance, Dispel Magic can be used so long as the caster can meet the same level of magic that should have been used in order to ensure a stable connection.
While Banishment can also be used as an alternative on the shadow itself, it is risky and not suggested as one must be certain which shadow is which. As there is no way to determine this, the use of this spell carries the risk of banishing the original soul and allowing the other to take over.
The Dangers Sadly, while this is a condition that can be easily incurred, it is not one that many know about as those who carry the accurate information are few and far in between. As the time of rifts is approaching and many more unstable openings are tearing through the veil, many of those who are knowledgeable are trying to disseminate the knowledge amongst clerics, paladins, adventurers, and all hoping that perhaps the word will spread and the shadows can be stopped.