Nithing-Staff

"'I will raise a nithing-staff and curse whoever did this, but I need a sword to do the work.'"
-SM Stirling, The Sword Of The Lady (New York, 2009), Chapter Eighteen, p. 557.

Questions
What is a nithing-staff? See here.
Surely cursing is a misuse of mental energy?
Does the Law of Threefold Return not apply?
If a sword is necessary for vengeance, then was the curse itself not effective?

A Nithing-Staff Is A Staff Of Scorn

This image, found on the Internet, depicts a scene described by Poul Anderson:

"When the wind came, [Egil] went onto a rock that jutted toward the mainland. In one hand he bore a tall, stout hazel staff he had cut. In the other was the head of a horse. Standing on the rock while the sea moaned below and a wrack of clouds blew out of a nearing rainstorm, he set the horsehead on the staff and lifted it upright. 'Here I raise a nithing-staff, and I turn its curse against King Eirik and Queen Gunnhild...'"
-Poul Anderson, Mother Of Kings (New York, 2003), Book Two, Chapter XX, p. 177.

The image also presents an alternative translation of the first line of the curse. Egil believes that this curse affects the local land-wights. He carves runes on the head. Gunnhild thinks that there will be two half-verses each taking thrice the number of all the runes won by Odin. She will respond with drum, song, dance, wilderness food and drink, power from the earth and the moon. Seid against runes. Primordial shamanism against Aryan sky gods.

A Horse's Head

See Nithing-Staff. (Scroll down.)

I am having a familiar experience. I remember reading a passage recently but have not been able to find it when looking back through Poul Anderson's The Golden Slave. Eodan says (something like) that, when he returns home to the North, he will raise a horse's head on a staff to curse Flavius.

We have encountered this kind of curse twice before:

(i) see the above link;

(ii) Eva Gabrielsson did it after Stieg Larsson's death although not with a real horse's head.

Blog readers have once or twice helped the blogger to find remembered passages.

Nornagest And Starkadh

Poul Anderson, The Boat Of A Million Years (London, 1991), Chapter V.

"'...it was a nithing's trick...'" (p. 115)

"How fared those masters, those fellow chelas?" (p. 126)

In this chapter, Anderson takes two legendary Norse heroes (see here), incorporates them into his small number of fictional immortals, imagines a meeting between them, then completes their stories in this single chapter so that they do not interact with any of his other immortals.

Each has lived for so long that he has heard of the other in stories. When Starkadh claims that King Frodhi never stinted him, Gest asks:

"'Was that Frodhi Fridhleifsson in  Denmark? They say Starkadh was of his household. But he died lifetimes ago.'" (p. 116)

Later, Starkadh shudders and mumbles:

"'Gest...I remember now, in my own youth there went tales of a wayfarer who - Nornagest. Are you he? I thought he was but a story.'" (p. 123)

When told that he is a figure in stories, Gest responds:

"'Often have I left the North for hundreds of years...So folk remembered me for a while, did they?'" (ibid.)

Yes, you have a Wikipedia article and appear in a novel by Poul Anderson. Because he has wandered East, Gest knows of Christ, Mohammed, the Buddha, Brahma and other pantheons, has learned meditation and has by now surpassed his gurus through many centuries of practice.