The doppleganger is a master of mimicry that survives by taking the shapes of men, demihumans, and humanoids.
Dopplegangers are bipedal and generally humanoid in appearance. Their bodies are covered with a thick, hairless gray hide, which gives them a natural AC of 5. They are, however, rarely seen in their true forms.
This monster is able to assume the shape of any humanoid creature between four and eight feet high. The doppleganger chooses a victim, duplicates his form, and then attempts to kill the original and assume his place. The doppleganger is able to use ESP and can imitate its victim with 90% accuracy, even duplicating the victim's clothing and equipment. If unsuccessful in taking its victim's place, the doppleganger attacks, relying on the ensuing confusion to make it indistinguishable from its victim. A doppleganger is immune to sleep and charm spells and rolls all saving throws as if he was a 10th-level fighter.
Dopplegangers work in groups and act together to ensure that their attacks and infiltrations are successful. They are very intelligent and usually take the time to plan their attacks with care. If a group of the monsters spots some potential victims, the dopplegangers often trail their targets, waiting for a good chance to strike, choosing their time and opportunity with care. They may wait until nightfall, or until their victims are alone, or even follow them to an inn.
Dopplegangers are rumored to be artificial beings that were created long ago by a powerful wizard or godling. They were originally intended to be used as spies and assassins in an ancient, highly magical war. Their creator died long ago, but they live on, still working as spies for evil powers, thieves, and government. They have even been known to work as assassins.
All dopplegangers belong to a single tribe. Although this is rare, groups of dopplegangers can be found anywhere at any time, and in unexpected locations. Working as a unit, they select a group of victims, such as a family or a group of travelers. Basically lazy, dopplegangers find it easier to survive and live comfortably by taking humanoid, and especially human, shape. They prefer to take the form of someone comfortably provided for, and shun assuming the form of hardworking peasants. Since they are only 90% accurate in their mimicry, most dopplegangers are eventually discovered and driven out, and then forced once more to assume a new shape.
Dopplegangers are found most often in their true forms in a dungeon or in the wilderness. Groups often set up a lair in an area well-suited to ambush and surprise, patrolling a regular territory. These bands make a good living by attacking weak humanoid monsters or travelers and stealing their food and treasure. If food and treasure are scarce, they hire out to a powerful wizard or thieves' guild.
A doppleganger who has been hired to replace a specific person will plan its attack with special care, learning as much about the victim and his environment as it can.
The dopplegangers' weaknesses are greed and cowardice. They spend their lives in avid pursuit of gold and other wealth. If attacking a group of adventurers, for example, they often choose the richest-looking one to attack first. If they target a party of adventurers, the dopplegangers wait until the party is on the way out of the dungeon and heading back to town. Since they are cowardly, however, they prefer to take the easiest route toward riches. A doppleganger who chooses a rich adventurer avoids risks once the treasure is safely in hand, and retreats at the earliest opportunity, making some plausible excuse for separating from the human members of the group. They sometimes hire out as spies and assassins for money as well.
Dopplegangers are sophisticated and dangerous parasites, living off the labors of others. They must also be reckoned with as clever and effective spies and assassins who can wreak political mayhem in positions of power. Evil wizards have on rare occasions controlled entire kingdoms for short periods of time by replacing a king, prince, or councilor with a doppleganger.