After Vaeroloth was cast into the Abyss, sealed into everlasting torment, the Maker further locked her away from his Creation by inverting the Void and separating this inversion from the natural Void by a thin and wispy "aleax" realm. This realm, also known as the Sliver of Silver, serves as the mirror image of the created realms, and provides a second layer of protection from Vaeroloth's attempts to influence the Creation for evil.
Reflections seen in the world, whether from bodies of water, polished steel, or man-made mirrors, are possible only because of the Sliver of Silver running throughout Creation. It touches all world and all realms, lending its reflective qualities to bright or tranquil surfaces.
But this mirrored realm also provides a means of transport, a practice it has been determined is extremely dangerous. Access to the Sliver of Silver is possible only by stepping through mirrors or reflective pools that have been specially prepared through powerful magic, opening them to anyone willing to enter this aleax world.
Once within the Sliver of Silver, things appear normal although slightly "off." Further interaction and inspection reveals that literally everything in the aleax world is, in fact, a mirrored copy of the natural world. Pictures are backward, room layouts reversed, left is right, and everything is opposite its normal form. While this can be disorienting initially, the larger danger is in encountering mirrored creatures, especially the mirror image of oneself.
Collectively, these mirrored creatures are called Aleaxes, and they are created simply by the viewing of oneself in any reflective surface. As might be expected, the Aleax is a reversed copy of the original creature--hair parted on the opposite side, moles on the other cheek, etc. But other opposites are unseen: right-handedness become left-handedness, lopsided grins sneer the opposite direction, and, more insidiously, one's very character is reversed. Good becomes evil; morality is swapped for immorality; ethical behavior is replaced with unethical. In nearly every way, an Aleax is the complete opposite of the Natural.
Once a mirrored surface has been opened for travel, it is guaranteed that one will immediately encounter one's own Aleax inside the Sliver of Silver, and it will result in a fight to the death. An Aleax cannot abide sharing its space with the original and will focus all efforts on eliminating the original. If an Aleax kills the original creature, it is free--nay, compelled--to enter the natural world through the open reflective surface, thereupon assuming the continuing life of the original as an imposter. If the Aleax dies, the original creature is free to continue travelling unhindered through the rest of the Sliver.
Travel through the Sliver of Silver is as easy as travel in the natural world, bearing in mind that directions are reversed. Leaving the Sliver of Silver, however, is quite difficult, and travelers have been known to be stuck behind mirrors for years, giving rise to the occasional ghostly image sometimes seen lurking in the background of a reflection. Many people cover mirrors when not in use to prevent this disturbing haunt, and if a particular mirror proves to be a favorite portal for an entrapped traveler, that mirror will often be shattered, forcing the traveler to move along. For the same reason, reflective pools are often stirred before someone will enter, just to make certain there is no possibility of accidentally interacting with something from the other side.
Significantly cracked mirrors and those with the reflective material flaking off the back are rendered useless as portals to the Sliver of Silver. Even the slightest ripples in a body of water break the connection and make the pool incapable of transporting someone to the aleax realm. Incidentally, mirrors and still pools within the Sliver of Silver have no reflective qualities whatsoever. Instead, they allow viewing only into the natural world. Passage through these into the natural Creation is only possible if the mirror or pool has been specially prepared from the aleax side, as is done on the natural side.
All but the most capricious of arcanists have regarded mirrors as a thing to safeguard and ward. Few are willing to open one and face their Aleax.