A doppelgänger hails from Vioo, the barren, dark realm that exists on the other side of mirrors and mirror-like surfaces (which form the only passages through which light passes into Vioo). Most of the time, doppelgängers have no life apart from acting as reflections for whomever is facing the mirror (which isn’t to say that all reflections are doppel- gängers). These monsters hate their limited existences. They hate the fact that they fade in and out of being, and that their appearances change, and doppelgängers most of all hate the people their strange state of being forces them to mimic.
Rarely, a doppelgänger’s hatred becomes so great that the monster manages to transcend the limitations of its nature. When this happens, the doppelgänger can act independently of the person using the mirror, and the doppelgänger can even break through the mirrored surface to enter our world. Then, the doppelgänger seeks to murder and replace the person it almost perfectly duplicates.
Doppelgängers have enormous physical strength. As such, they enjoy a +2 bonus to attack rolls, and their weapon attacks (including unarmed attacks) inflict an extra 1d6 points of damage. These creatures are immune to sleep and charm, and they are considered magic resistant for purposes such as breaking through wizard lock and similar spells. They have a very good saving throw (5+) against magic of all kinds.
Swords & Wizardry
Hit Dice: 4
Armor Class: 5 
Attacks: By weapon
Saving Throw: 13 (5 against magic)
Special: Enormous strength, immune to sleep and charm
Move: As humanoid mimicked
Challenge Level/XP: 6/400