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.

They say that the "Faerie" race were the sublimated people known in Eire as the "Tuatha De Danaan" the "Children of the Goddess Dana or Danu. I think Robert Graves in his "The White Goddess" proposes an origin in Thrace for these people, who he thinks traveled up the Danube, named after the Goddess, to Denmark (also eponymous) and then across to Britain (then called Samothea) about 1800B.C, and then to Ireland, where they displaced an already ancient aristocracy. The "Sons of Mil" or Milesians displaced the Tuatha De Danaan in their turn, and it was said that they took to the "hollow hills" meaning the "Raths" or clan hillforts, and we can probably say that something of the sort must have happened here in Albion. The Tuatha were supposed to have inherited great magical powers which they were obliged to use against their sworn enemies the Fomorians, who were banished beneath the Western Sea. The association with Ley lines, water & wells, dingles and hidden glens, and round barrows is well known for water dryads and other faerie spirits. This may indicate not so much that they are integral within the land but that at some point (and perhaps at some times) human perception and intuition, timelines, and the landscape interpenetrate one another to produce contact and connection. Some suggest that the Sons of Mil were the Celtiberians, and if it wre the Celtic incomers who overwhelmed the Fearie race, they would have probably incorporated folk memories of their predecessors in stories and myths.