Shadow

Shadows are undead that resemble dark exaggerations of humanoid shadows.  The most discernible feature of a shadow is dark grey skull-like shape that rests in its head.  However, this skull is no more solid than the rest of their body.  
From the darkness, the shadow reaches out to feed on living creatures' soul energy.  Their attacks affect the animus of living creatures, weakening the connection of the soul and body, causing lethargy and eventually death.
They can consume the animus of any living creature, but they are especially drawn to creatures untainted by evil.  A creature that lives a life of goodness and piety consigns its basest impulses and strongest temptations to the darkness where the shadows hunger.  As a shadow drains its victim's strength and destroys its animus, the victim's shadow darkens and begins to move of its own volition.  In death, the creature's shadow breaks free, becoming a new undead shadow hungry for more life to consume.  If a creature from which a shadow has been created somehow returns to life, its undead shadow senses the return. The shadow might seek its "parent" to vex or slay.  Whether the shadow pursues its living counterpart, the creature that birthed the shadow no longer casts one until the monster is destroyed.
A shadow doesn't require air, food, drink, or sleep