Look at page 70 of the Shaman's Handbook.
Possessing Fiend.
Although its nature has caused it to "Evolve" unique spirit/ some unique powers. It gains power each cycle it possess and kills progressing in Hit Dice and Abilitites until some proscribed event occurs . For example, it takes a yound man. Then porceeds to kill according to its nature ( Priests, acts of selflessness, young woman, virgins of either sex, criminals, rapists...whatever needs to be determined ), gaining hit dice, advancing as it kills. Then some event occurs, ( witnesses a selfless act, is Named by having the victim identify the host as what it is, a powerful turning, failing to kill a loved one of the host by proscribed time) causing the fiend to be dispelled and de-invested of its gained power. Only to be called forth by its "Trigger" ( Nameing it in Fear, Worship, spilling blood of an innocent in a certain alley at a certain time. etc.)