They go by many names--hag, succubus, night elf, night goer, etc.
They prey on people when they are most vulnerable, when they sleep and then they feed on your energy. They're vermin of the spirit world. They only seem to come out at night. There is tons of protection/amulets against them. Basically any good protection will do. Sleep with a protection under your mattress. A piece of iron (like an iron nail or hematite which is iron ore) blessed and soaked in a brine is a good ward. "Holy" items and prayer doesn't effect them because they're not "evil", they're of death/illness/fear.
It doesn't mean they're going to kill you, but people do tend to feel weak, drained, anxiety ridden or sick when victimized. Only when they prey on the sick or dying does death become a concern.