The Coldfells was a region in northern Eriador, on the foothills of the Misty Mountains.
Location
The Coldfells were located "north of Rivendell", but they are not mentioned on any map (or anywhere else, for that matter). Circumstantial evidence suggests the Coldfells were near to or synonymous with the Ettenmoors, which are described as "the troll-fells north of Rivendell" or the Ettendales, which are described as "far north or Rivendell" and to be "troll-country".
In Third Age 2930, hill-trolls captured Arador, the Chieftain of the Dúnedain, in the Coldfells and slew him. Arador's son Arathorn II succeeded him, one year before the birth of the last Chieftain.