A witch, any witch, is a woman who has made a blood and soul pact with a outsider like a demon, devil, starspawn, or even the fae. The cost of this transgression is death, and the burning of the corpse so that it cannot be raised again by it's eldritch master.
A Doppleganger Witch is a specific type of witch who has mastered a sphere of magic devoted to emulating the appearance of others, creating duplicates of themselves, or others, and acting as a destructive force inside society. These witches are very dangerous because it is easy for them to infiltrate and subvert a city, fortress, or even a circle of Adventurers-Upon-Return.
How to Detect a Doppleganger Witch
Detecting a Doppleganger Witch is difficult, and the most common method is simply recognizing a face that a person has seen more than once, such as seeing the face of a hospitaler nurse in a group of erstwhile heroes, or seeing a servant who shares features with a witch that has been met in the past. For a Doppleganger witch emulating another person, observation remains key. The emulation is never perfect, and they will either display a lack of certain mannerisms or knowledge that their target would know, of their appearance will have a discrepancy, such as the turn of the hair, or some aspect of their bearing.
Watch for Witch Tells
The next most common method is to seek out the witch's tell. Do animals become uneasy in their presence? Does food spoil rapidly, especially milk curdling? Is the wine turning sour without explanation. The use of a silver mirror can either show the witch's true seeming (a dangerous test because a revealed witch could fully revert to their witch form and engage in magic mayhem) or their reflection could move and act of it's own accord. A witch's shadow can likewise be observed. If a witch is suspected, a silver coin or silver holy symbol can be tossed in their shadow, and if they are a witch, it will immobilize them. As with the mirror test, this can cause a concealed witch to reveal themselves with full magical violence.
The Psilogists of the Witch Hunting orders have identified a number of spells that a witch will use to carry out being a doppleganger
Prismatic Duplication - a lesser form of duplication, this spell creates a number of duplicates, either of the witch or of her target, all remain under her control, and at any time, she can manifest her will, senses, and magic through one of the duplicates created. Each duplicate will have an incomplete and emotionally tinted version of the original, with a single attribute of cluster of attributes brought to the fore front. The most commonly is the manifestation of a good duplicate, and an evil one. There are also accounts of duplicates who are cowardly, excessively prone to violence, overcome with malaise, consumed by lust, and so forth. These duplicates are easy to spot, and do not work well for long term infiltration.
Greater Prismatic Duplication - like the lesser form of the spell, this form creates near perfect duplicates, with the modified temperament. In the lesser form, the duplicates can often be buffoonish in their actions and appearance. In the greater form, their resemblance to the original is much greater, and harder to detect. Witches who use the greater form to duplicate themselves can do a lot of damage as many of the duplicates can adopt widely differing appearances and mannerisms.
Ghost Doppleganger - this spell creates a spectral image of the target, but it has no substance, cannot make sound, and has neither a reflection or a shadow. The witch creating it can send her senses through it, but cannot control the phantasm beyond her range of sight. This can be used to spring ambushes, surprise unsuspecting hunters, provide a diversion, or disrupt a hunt in progress.
Lesser homunculus - the witch creates a copy of herself or another, and it acts as if it is the person in question, not knowing it is a duplicate. Many times a witch has escaped by creating one of these decoys to be captured and executed in her place. A rite of magical banishment will cause a homunculus to wither away into its component materials, but these rites require time and a dedicated site, and many consider transporting a captured witch to such a sanctuary to be too dangerous and will carry out execution and cremation on the spot.
Greater homunculus - as the lesser form, but all but indistinguishable from the original target. A greater homunculus can be weaponized with spell like powers, the ability to reshape it's form, and can have attributes far beyond what a mortal should have. It is not uncommon for a discovered greater homunculus to alter its form to that of a known monster or a horror inducing Gibbering Mouther to better and more freely move and kill. These creatures can assume the form of any victim they have slain and absorbed, including the appearance of their weapons and equipment. Smart homunculi will save these items to not have to replicate them with their bodies, plus, a long sword made of their flesh is still just made of their flesh and not actual steel.
Lesser Duplication - this spell creates a copy of the target.This duplicate is several levels lower in capability, as well as having depressed mental and spiritual attributes. It retains freewill and can live indefinitely on its own, but it will crave to return to the witch that created it and either serve or be reunited with her. When a witch reabsorbs a lesser duplicate, she gains all the memories and experience that it had gained while away.
Greater Duplication - This spell creates a duplicate as lesser duplication does, but with only slight decreases in level and ability. A greater duplicate of a magic user can still use the magic that they would know, again, at a decreased level. This spell is more difficult and it not done lightly. Greater duplicates are less needy, and less inclined to fuse with the witch that created them.
True Duplication - one of the most terrifying spells a witch can learn, this spell allows her to create a perfect a true duplicate of herself. This creates two witches of equal power and equal magic ability, and functionally every duplicate created IS the witch. With the lesser versions of the spell, there always remains a single prime witch, the original. With this spell, every witch created IS the prime and the original. The spell is very expensive, and afterwards all the witches created by it are depleted and exhausted.
Dangers of the Doppleganger
The traditional witch is a danger to everything around her, summoning monsters, casting charms, and creating pockets of decay and ruin around her. This can be limited, besieged, and she can be brought to earth, judged, and burned. The Doppleganger is a viper who can enter a holy city, blend into a group of adventurers, become a patron of heroes, sending them into traps and dungeons to meet their doom, and to drain the coffers of a kingdom, bedevil it's men, and to bring a nation to its knees.