The motif The grave-pig is a part of: Ghost
See also The bone-horse
Death, ghost, portent, sign, omen
Just at that moment the elf mound opened, and an old-maid elf minced out of it. The woman had no back, but otherwise she was quite properly dressed, with her amber jewelry in the shape of a heart. She kept house for her distant cousin, the old king of the elves, and she was very spry in the legs. Trip, trot, away she went. How she hurried and scurried off to see the night raven down in the marsh.
"You are hereby invited to the elf mound, this very night," she told him. "But may I ask you to do us a great favor first? Please deliver the other invitations for us. As you have no place of your own where you can entertain, you must make yourself generally useful. We shall have some very distinguished visitors-goblins of rank, let me tell you. So the old elf king wants to make the best impression he can."
"Who is being invited?" the night raven asked.
"Oh, everybody may come to the big ball-even ordinary mortals if they talk in their sleep or can do anything else that we can do. But at the banquet the company must be strictly select. Only the very best people are invited to it. I've threshed that out thoroughly with the elf king, because I insist we should not even invite ghosts. First of all, we must invite the old man of the sea and his daughters. I suppose they won't like to venture out on dry land, but we can at least give them a comfortable wet stone to sit on, or something better, and I don't think they'll refuse this time. Then we must have all the old trolls of the first degree, with tails. We must ask the old man of the stream, and the brownies, and I believe we should ask the grave-pig, the bone-horse, and the church dwarf, though they live under churches and, properly speaking, belong to the clergy, who are not our sort of people at all. Still that is their vocation, and they are closely related to us, and often come to call."
Female elves have hollow backs.
The night raven, which is a bird characterized by strange screams and silent flight, has in folklore become an exorcized ghost, cf. jvf. Flemming Hovmann's comment in vol. 7 of H.C. Andersens eventyr, Dansk Sprog- og Litteraturselskab / Borgen 1990, p. 100.
The church dwarf is a pixie, that maintains order and correct behaviour in church kirken. In Thiele's Danmarks Folkesagn (1843-60), 1968, vol. 2, p. 219ff, church dwarfs fight to keep ghosts of drowned bodies at the beach away from the church.