Doppelganger

In its true form, which is rarely seen, a doppelganger is a humanoid biped with thick, hairless gray skin that gives it a natural AC of 5.

It can take the shape of any humanoid from four to eight feet tall. Its method is to choose a target, copy its form, kill the original and step into its life. With ESP and mimicry 90% accurate, it reproduces even the victim's clothes and gear. If the swap fails, it attacks anyway, counting on the confusion to make it impossible to tell which one is real. Sleep and charm spells do not affect it, and it saves as a 10th-level fighter.

Highly intelligent, these mimics hunt as a team and plan carefully. A band that spots likely prey shadows it and waits for the right moment, perhaps nightfall, a time when the victims are alone, or a night at an inn. One hired to replace a particular person studies the target and its surroundings exhaustively first.

Rumor says they are artificial beings made long ago by a mighty wizard or minor god as infiltrators and killers for an ancient war of magic. Their maker is long dead, but they carry on, spying for evil powers, thieves and governments, and sometimes killing for hire.

All doppelgangers belong to one tribe, and small groups can turn up anywhere, often where least expected. Together they choose a cluster of victims, perhaps a household or a traveling party. They are lazy at heart and find humanoid, especially human, disguises the easiest way to live well, so they copy the comfortable and avoid hard-working peasants. Because their copies are only 90% accurate, most are eventually found out, driven off and forced to start over in a new shape.

In their true forms they are most often met in dungeons or the wilds, where a band lairs near a good ambush site, patrols a set territory and lives off weak humanoid monsters and travelers, taking their food and treasure. When pickings are slim, they hire themselves out to thieves' guilds or mighty wizards.

Greed and cowardice are their flaws. They chase wealth relentlessly and usually single out the richest-looking member of an adventuring party, striking as the group heads home from a dungeon. Yet they always take the easiest path to riches: once one has the loot in hand, it avoids danger and leaves at the first chance, with a believable excuse for parting from its human companions.

Doppelgangers are cunning, dangerous parasites that live off others' work, and as spies and assassins they can sow political chaos from high places. On rare occasions evil wizards have briefly ruled whole kingdoms by swapping a king, prince or councilor for one.