A vengeful spirit (怨霊 onryou, also "evil spirit") is a special type of ghost created under special circumstances and is the most dreaded type of ghost possibly encountered. These are the spirits of people that died of such strong passions – be it hatred, jealousy, or anger – that their spirit is unable to pass onto the afterlife and instead transforms into a wrathful spirit that will seek vengeance on everything and everyone it encounters. Their appearance will look exactly like it was in life, and they will often display marks that indicate how they died.
Vengeful spirits are powerful enough to be able to kill someone in the blink of an eye, but instead, they'd rather watch the object of their hatred live a life filled with suffering of torment, watching them suffer and die. Vengeful spirits can do this by inflicting a curse on the people and places they haunt. Said curse can also be spread to other people, like a contagious disease, which creates a devastating circle of death and destruction that no other type of ghost is capable of inflicting. Not only do these ghost not make distinctions in who they target (their goal is simply to destroy), but their vengeance can never be satisfied like most other spirits.
Unlike a typical ghost who will only haunt a person of place until it is placated or exorcised, a vengeful spirit's curse can live on even after the spirit has been laid to rest. Sometimes, these spirits aren't born from grudges, but from intense passionate love which quickly perverts into extreme jealousy. These types of ghost will haunt their former lovers, their new romances, second marriages, their children, and destroy the lives of their former lovers. It's no wonder these types of spirits are the most feared supernatural entities in Japanese culture.
Vengeful Spirits in the Len'en Project
Taira no Masakado is a vengeful spirit that Taira no Iwakado planned on resurrecting, even going as far as to use his own grandchild, Taira no Fumikado for his plans. He used the body of an oni child that he hunted with Fumikado's blood, adding the vital energy and some special qualities from a lot of other living things to manufacture a new life-form. Feeling grateful for letting him take care of them, Fumikado agreed to work as his medium and take part in those experiments and even transferred their soul into the body to create a homunculus. The only thing left was to transfer Masakado's soul, but an important partner was killed by anti-Masakado's forces, so Fumikado had to abandon the Capital City before Masakado's soul could be transferred into their body.