Doppelganger

Crafty.  Shape-shifting.  German (although somewhere along the way they lost the umlaut).  Not to be trusted.  At low levels they make for excellent solo assailants (see the notes below); at mid- and high levels their machinations can still make PCs’ lives miserable—abducting and replacing their friends, framing them for felonies, and otherwise creating an atmosphere of paranoia to hamper their every move.

A gang of doppelgangers insinuates itself into the city.  Its leader is obsessed with spell completion items, and the fact that it is never without a wand or three may be the tip-off needed to unmask it.

Some advanced doppelgangers have limited telepathy or can adopt the memories of their victims.  When a famous oracle is revealed as a doppelganger, it is unclear how long the deception has been going on, or if the doppelganger itself even knew its true nature.  Apparently exposure to the original oracle’s haunted affliction has permanently twisted its mind.

Doppelgangers’ laziness and desire for wealth and luxury are not unique traits among shapechangers.  Rakshasas and certain oni in particular pursue similar ends—and they don’t like to share.  As sea trade grows among the nations, these monsters are increasingly coming into conflict.  Janissaries up and down the Serpent Coast report stirrings of a Shifter War, with dueling morphing and magical beings duking it out in the shadows night after night for territory.

—Pathfinder Bestiary 89

Back in the day there was an amazing Dungeon mystery featuring a doppelganger trying to take over an inn.  Anyone remember which issue?

I can’t believe I’m about to reference Star Trek (before this springs I’d barely ever watched any; I grew up in a Star Wars house), but any episode of Deep Space Nine with the Dominion will likely hold a nugget or two of inspiration for making your doppels truly wicked and manipulative.

I prefer the shriveled, gray nosferatu-looking doppelgangers of “basic” D&D to the grey-alien-looking beasts that are currently in vogue, but too each his own.  Also, while not about doppelgangers per se, Races of Eberron is worth checking out for seeing how that setting’s doppelganger-derived changelings handle multiple identities in the world.