Doppleganger Demon

Combat: in hand to hand figth, the doppleganger demon attackes three times per round, inflicting 1d3 damage with claws and 1d6 damage with its bite.

The most dangerous attack of the doppleganger demon, however, is the life drain; for every wound the victim receives, he or she loses 1 point in Intelligence. If intelligence falls to zero, the victim dies and the demon can assume his form, knowledge (proficiencies etc.) and all of his special skills (spells, multiple attacks etc.), however the demon's hit points and hit dice aren't altered even if the special powers drained from the victim are used as if it were of the same level of the victim. Additionally, for every three Intelligence points it drains, the doppleganger demon gains or recovers one HP; it is possible with this power that the demon goes beyond its usual hit points maximum, in that case, however, the bonus isn't permanent, but it only lasts for 1d10 turns. Lost intelligence is regained only when the demon dies.

It is very difficult to reveal the true identity of the demon when it assumes the shape of a creature. Only True Sight, Limited Wish or Wish (or psionics of the same power) can succeed in this task; in the first two cases, however, the doppleganger demon can attempt a saving throw at +4 to avoid being discovered, while a Wish never fails. All other detection spells or psionics will always give a wrong answer, without the caster or psionicist realizing it (the doppleganger will always appear as the creature it is mimicking, without doubt).

The doppleganger demon can emit a beam of light from its eyes once every three rounds: the effect is the same as the Light spell. This power can be used simultaneously with the other attacks (spells, claws and bite, etc.). Additionally it can use the following spells three times per day as a 10° level caster: Detect evil/good, ESP, Detect alignment, Vampire Touch, Darkness, Dispel Magic.

The doppleganger demon only suffers half damage from nonmagical weapons, has 20% magic resistance, uses the saving throws of a 10th level warrior/mage, uses thief skills as if it were 15th level and is immune to electricity and gas; cold inflicts half damage while fire inflicts one more wound per die of damage.

A holy water vial will inflict 2d4 damage to a doppleganger demon while a holy weapon, blessed by a lawful good, neutral good or chaotic good  god, will inflict respectively 1d4, 1d3, or 1d2 extra damage.

Habitat/society: these beings, with their powers of disguising, are used by fiends as spies. They don't have a social organization and many of them are loners who wander the Prime Material Plane looking for people to drain. When its presence is needed, the doppleganger demon is recalled by the superior fiends, who trust him delicate tasks in which knowledge of the art of deceit is very important.

Ecology: the doppleganger demon feeds on the life energy of its victims but sometimes has a snack with their flesh, just for the taste of it. Little is known about them, but they are very feared, for their exceptional skill at disguising as other people rather than for their strength.

Source: Fabio.