Strige

The exact method of becoming a strige is unclear, though there is likely more than one, and a divo is involved.

You can tell someone is a strige by one or more of the following: a double row of teeth, extra finger or toe, two hearts, a spare head, a suspiciously-shaped birthmark or scar, inconsistent or manic behaviour, unnatural coldness, lack of empathy, bouts of extreme violence, tendency to wander around on moonless nights, enhanced sense of magic and spirits, an evil sort of face, frequent nightmares...   There are rituals and magics that can be used to determine whether or not someone has more souls that strictly necessary; divy and some other magical creatures can also tell. For those without distinct physical markers, the strige will often know what they are before anyone else -- the presence of a second soul/self in their mind can only be ignored for so long, even if in most cases it will remain dormant until the body dies and the first soul leaves.

The only known and proven way is a divne intervention. Rituals exist to bind the second soul, but they only last for a few hours, a day at most, and must be done within moments of death. Destroying the body by burning is the only sure-fire way to prevent a strige from rising.   Once risen, there are a few ways to break it. Blessings and name-bestowing are said to be one, though it is hard to get the strige to cooperate. Some say that spending a night in the strige's burial/death place, while it is out hunting, will confuse it enough to weaken the bond between its body and soul. Dismembering works as well as it works on anything, though it should be noted that strige will not bleed out and are incredibly strong and nimble. Enough fire will always work -- when it doubt, burn it! But scatter the ashes, just in case.

There are little to none changes during the normal day-to-day life of a striga/strygoń; the main effect comes up when they die. Within a few hours at most, the second soul rises to finally take control that has been denied it. They thrive on blood and spirit, and retain few, in any, sentimental/emotional or logical connections of the first soul. A striga's body is weaved and tangled in with spirit magic to keep it from falling apart and decomposing; this also makes it must stronger than even a strong human.

Any being with a soul separate from their physical body; so humans, but also some animals and spirits.

Congenital version: There is no known way of preventing a person from becoming a strige. Several rituals and spells are advised to expecting mothers of course, but no conclusive proof that any of them work. Mostly they focus on keeping potential malicious magics and spirits away from the household during the pregnancy. Since pregnancy lasts so long, this is basically impossible.   Acquired: It is advised that people do not perform dark, soul-destroying rituals on themselves. It never ends well.

The congenital version seems to be random, perhaps triggered by a strong spiritual/magical interference at a point between conception and birth. The acquired version is a result of more deliberate action: a consequence of a ritual gone wrong, or a curse.

There are two major paths the life of a strige takes, once it is known what they are.   * They are feared and ostracized, often imprisoned and, once a safe method is agreed upon, executed to prevent the second soul rising and rampaging. Rarely they are allowed to live out their natural lifespan; there was a case of one, placed under a restrictive house arrest, taking their own life deliberately, as means of revenge upon their jailors.   * They are given to a powerful divne. The divne can destroy the second soul, though it usually cannot do it without also destroying the first one (that part is left unsaid, and is in fact unknown to most). It does it by basically taking over the host body: both souls are displaced and dispersed, or incorporated into the divne itself.