They aren't praying for the dead. They are trying to suck the spirits out of these dead people, hence the name "grave sucking".
"Necromancy /ˈnɛkrɵˌmænsi/ or nigromancy is a form of magic involving communication with the deceased – either by summoning their spirit as an apparition or raising them bodily – for the purpose of divination, imparting the means to foretell future events or discover hidden knowledge, or to use the deceased as a weapon, as the term may sometimes be used in a more general sense to refer to black magic or witchcraft."