Poweries (also called Dunters) are said to be similar to a redcap, so to start, I feel like I should first explain what a redcap is.
With his short height, a redcap might seem like a brownie or gnome, but instead of being a friendly helper, the redcap is mentioned as cruel.
“He is depicted as a short thickset old man, with long prominent teeth, skinny fingers armed with talons like eagles, large eyes of a fiery – red colour, grisly hair streaming down his shoulders, iron boots, a pikestaff in his left hand, and a red cap on his head.”
“Notes on the folk-lore of the northern counties of England and the borders”
by Henderson, William (1879)
The redcap haunts ancient castles and towers, and is very territorial. So territorial, in fact, that he may resort to murder.
“When benighted or shelterless travellers take refuge in his haunts, he flings huge stones at them; nay, unless he is much maligned, he murders them outright, and catches their blood in his cap, which thus acquires its crimson hue.”
“Notes on the folk-lore of the northern counties of England and the borders”
by Henderson, William (1879)
“Gnomes and mushroom picker in the forest” by Heinrich Schlitt
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Now, back to powries/dunters. They are said to be similar to the redcap, but with two very obvious differences; they do not have red caps, and they are said to be very noisy creatures at all times.
“They make a constant noise, like beating flax or grinding barley in a hollow stone quern.”
“A Encyclopedia of Fairies” by Katharine Briggs (1976)
Much like the redcaps, these creatures haunt specific buildings, and it was suggested by William Henderson in his book Folk-Lore of the Northern Counties(1866) that they are the spirits of those — human or animal — who were sacrificed at pitish location.
There is not archaeological evidence to support this at all the haunts, so others just say that powries and dunters are simply territorial fairies that claim ancient buildings.