The above named author writes, in his 1888 publication-"The Comprehensive Guide to the County of Northumberland" (1985-Davis Books) That---" The Simonside hills are said, by tradition, to be haunted by a species of mischievous elves called the Duergar" (p338) and goes on to relate a tale about how they set the water wheel at Tosson Mill a-going at night.
Another story tells how the Duergar led unwary travellers into bogs and and slough`s.
I belive that, as the inhabitants of Nortumberland were Brythonic Celts, and therefore speakers of P Celtic, or Old Welsh, the word "Duergar" contains the element, "Dwr" ( Welsh-Water) and that these were water spirits, as the association with water in the above stories show. The Welsh word Dwr is pronounced the same as Duer in the English tongue.
Gwalchmai ap Taliesin.