Shadow is a virus that infects its host and possesses it to become one with the shadows, it is a major problem because it infects at a rapid rate. Excalishman is currently trying to contain them so they can no longer spread to others.
Shadow first came to Zmaria in a meteor containing a black liquid, a fire fifi touched it and the black liquid started to cover its body. He then received a set of enhanced powers and its mind became much much darker. Shadow can now be found scattered all across Zmaria, though thankfully mostly in small amounts. However, as it spreads at a rapid rate, it can quickly overtake huge regions if not contained. A perfect example of this is the Infinity Tower. No one knows who built it, but it is known it was originally built to contain a number of shadow-possessed fifis. Many adventurers have fallen to the shadow while trying to reach the top of the Infinity Tower.