Living Shadow

Long, dark fingers stretch out across the wall, reaching toward light, life, and all that they do not have, but long to possess. The shadows move and hunger, for their very essence is gluttony and greed.
Rampant covetousness and grasping greed lead some people down the dark path of evil and betrayal, eventually ending in a reprehensible death scene or a lonely expiration. While most such petty and despicable souls travel on to their final rewards the same way everyone else does, in some cases gluttons, misers, and thieves waste away into nothing but shadows—undead things that reach and grab but cannot hold. Over time, spellcasters have discovered that these avaricious souls make perfect servants and guardians, immortal creatures doomed to watch over “their” treasure for all eternity.
Shadows are incorporeal undead, distorted like their namesakes and able to float or slide silently along surfaces, blending in among the true shadows there. This allows them to approach unnoticed, and those trespassers not caught completely by surprise rarely get more than a glimpse out of the corner of an eye—a flicker of movement and the sense of something out of place—before they strike.
In addition to guarding the haunted ruins they lay claim to or serving more powerful undead capable of cowing them, shadows devote themselves to attacking any living creatures they encounter, draining their victims of all vitality with their chilling touch. Victims become weaker and weaker until they finally perish, but their suffering is only beginning. For as the victim of a shadow’s touch expires, its own shadow detaches from the corpse, taking on the same half-life as its killer, hungry for the essence of the living and operating under its killer’s command.
Like all undead, shadows are timeless creatures. As they have lost all concept of their previous life in the transition to undeath, the passage of centuries means almost nothing to them, and no one can say what shadows may do or think in the long wait between victims. However, unlike lesser undead, shadows do appear to “grow” over time. A shadow that has fed on the lives of many victims, or that dwells long enough in a place suffused with sufficient negative energies, may grow in power, becoming a greater shadow. These “shadow lords” often command swarms of their lesser kin, typically spawn of their own making. Rarely is more than one greater shadow found in a particular place, as the creatures compete fiercely for prey. Some believe that in especially fallow times, shadows even consume their own, but this is almost certainly false, as consuming other undead would grant a shadow neither the energy it seeks nor a new spawn, and gangs of shadows have been found that survived sealed into lost tombs together for millennia.
What awaits powerful shadows is a question even the sages can only speculate about. Some believe shadow lords may eventually become shadow demons, drawn down into the Abyss by the weight of their sins to drown in the eternal darkness. Torn apart by the forces of chaos, they emerge as malevolent monsters of pure envy and avarice. Others claim shadow lords steal the vitality of the living to become more corporeal, eventually transforming into other undead creatures or half-real shades.

( source: http://www.d20pfsrd.com/bestiary/monster-listings/undead/shadow )