The animated shadow is by definition a being of lesser standing, an envious follower, dependent on light and an intervening body, aspiring to an independent state it can never have. Like the ghouls, its status as undead wavered in early D&D, and is still uncertain in (for example) Swords & Wizardry. There is a certain eerie power in the idea of a shadow that one day refuses to serve, and that really doesn't need the additional tag of undead to chill the blood. Lord Dunsany memorably explored the idea of a detachable and tradeable penumbra in his novel The Charwoman's Shadow.
The sin of envy, or the medieval invidia, is studied by psychologists and philosophers as an emotion. It seems to be a pretty common theme in such studies to split the emotion into a "good" side and a "bad" side, which reflects the idea that emotions are basically functional but often go wrong. In this research, benign envy is explained as a desire to emulate the superior, while malicious envy involves achieving parity through reducing the superior instead. It is the destructive side of envy that the evil shadow embodies.
Shadows make a fine undead creature. Think of them as a void of life-force, lacking entirely in Strength. When they see a being with superior Strength, they enviously approach, implying that they will attack the strongest by preference. Their touch destroys the strength, but does not steal it. Eventually, their victim ends up dragged down to their level, a shadow among shadows.
What of the person who voluntarily chooses existence as a shadow? This is a rare wish, supported by an equally rare necromantic rite, and marks the ultimate triumph of envy. To give up one's own worthless existence for the chance to tag at the heels of a far superior person is similar to the motivation to become a zombie, perhaps. Thus, the living shadow creeps at its master's feet in imposture of the natural shadow. But the creature saps, rather than serving, its superior - making one attack per day on the victim's abilities, and adding any stolen points to its own score, until it merges in hateful life with the victim, creating a being of walking dusk without a shadow of its own, and adding the victim's hit dice and hit points to its own. Thereafter the envious soul has no reason to remain, its victory complete, and the completed shadow-being tends to become possessed by a darker genius.
At both stages, the living shadow can only be turned by two or more clerics, each bearing a holy symbol on which a clerical light spell has been cast, and arranged so as to surround the shadow.
For this as well as the more common undead variety of shadow, there exist several varieties which each possess their own special attack upon the abilities, and their own consequence once draining is sufficiently advanced (1d3 points on a successful hit, regained at 1 point/day). The consequence of zero points in any ability is death and conversion to a shadow.
1. Attacks strength; at 1-2 points victim becomes flaccid and unable to move.
2. Attacks dexterity; at 1-2 points victim becomes hyperactive, moving and acting about uncontrollably.
3. Attacks constitution; at 1-2 points victim becomes moribund and takes 1 hit point damage each round of moving.
4. Attacks intelligence; at 1-2 points victim becomes amnesic and unable to remember declarative facts such as his or her own name.
5. Attacks wisdom; at 1-2 points victim becomes catatonic, unresponsive to outside senses and lost in a world of his or her own mind.
6. Attacks charisma; at 1-2 point victim becomes repulsive even to him or herself, and will attempt suicide if not restrained.