Idea for an evil spirit, which I am nicknaming Darklings for the moment.
Darklings are malicious predatory beings, which appear as fanged mouths on serpentine necks, which dissapear into darkness too thick to see into. They can only manifest in conditions of little or no light, they dissapear like the shadows they call home when exposed to bright light. Darklings are unable to physically affect any living being that has not killed another person, but they can be seen like any other creature and speak to anyone they wish.
Darklings know everything, and can only speak the truth. In theory this would be of great benefit to mortals, but the darklings speak in riddles and half-truths, only telling the full truth of any subject when it will lead the listener towards acts of violence against other mortals. Darklings willingly appear before the paranoid, the frightened or the angry, whispering to them of the things that will drive them deeper into dark emotions, hoping to drive them to acts of murder, after which the darklings can feast upon their pawn at will, slithering from the shadows to burrow into flesh and feed on the heart.
Darklings like to convince people their friends are conspiring against them, that their loved ones are having affairs, that business rivals seek to murder them, and other dreadful things. Sometimes the darkling is even telling the whole truth, it matters little to them. They delight in the mental anguish of their victim confronting betrayals real or imagined, supping upon every misinterpretated word, each misunderstood gesture.
Should the victim ultimately take their own life as a result of the darkling's words, rather than that of another, the darkling is able to infest their body. The possessed victim is returned to a half-life for a brief time, which the darkling uses to spread more mental anguish, writing letters from the deceased to the living, or bringing their host to a location where their body might be interpreted as a murder victim. Very rarely a darkling is able to truly possess their victim, restoring them to full life and taking up root in the back of their soul, guiding and influencing them for the darklings own sadistic pleasure until their body meets it's end once more.