All things eventually die. Some things die more slowly than others. Some magic refuses to die. Old magic, tied to ancient artifacts or places of power will do everything to prolong their life. The fading gasps of powerful magic will grow into an enervating force, shadows that come to rip the life force from those nearby, These shadows creep out, avoiding the light and attempting to find victims to claim.
These shadows are often thought to be ghosts or undead, but they are enervating magic brought forth from these things. Most of the time they are brought forth from a ritual site with a permanent magical effect. Many magical fortresses with defensive magic, magical laborotories, or sites sealing away evil will begin to create shadows to maintain the spell. Ancient magical items which are abandoned and forgotten will also begin to reach out and claim life to maintain their legend itself. Very rarely, sorcerous spells gone awry (typically Tax/Arcana) will form shadows to reach out and consume everything around.
Shadows are created very slowly, typically with one or two being created a forte night by a magical site (depending on the power of the site itself, very powerful sites will create a second shadow). Magical artifacts will create one shadow per year per Obstacle to create on the Enchanting skill, though it's rare for an item with less than Ob6 to ever gain enough will to create a shadow. If the Wheel of Magic ever rolls Anima and Tax there is a chance it will create one or more shadows, and an Ob6 Ritual roll can bind those Shadows to a spellcaster, though they need to be kept in place for the hours it takes to perform the ritual. Some liches and other strange spellcasters have been known to create shadows as a method of prolonging their unlife, but it is unknown how these are created.
The benefit to the magical area of the shadows is simply longevity. The shadows touch will drain life from the victim, and once the shadow has managed to drain enough life it will attempt to return to its source, to place that life into the spell itself. Those attempting to find and stop shadows will often have an animal go ahead of them and then attempt to follow the shadowy form (no easy task, since they travel only in the dark and are nothing but splotches of darkness themselves) back to the shadow's source, in order to destroy it. Of course, finding out how to destroy the object or ancient ritual is often an entirely new problem. If the source is a lich or other undying monster sending horrible minions to find lifeforce, it might be a death sentence.
Made of Darkness: This creature is made of darkness. Its Stealthy is one shade lighter. When it goes into bright light, it receives a B8 wound and begins to smoke and wither. Torchlight or other light sources can be used to harm the Shadow, use the lightsource as a guide, a bonfire might be a B6 wound whereas a torch might be B3 wound unless stabbed directly into the shadow (where it would probably be B5).
Enervating Touch: When the shadow uses its Terror power it begins to collect life essence. Each shadow wishes to deal 5D of wounds, and then once it has done so it goes back to its source to return the power.
Terror: As per Spectral Echoes
A Burning Wheel Monster Every Week