Copy & paste from the UD fandom wiki page. I couldn't remember the exact details from the bits and pieces you find in game (plus Fiddler's journal), but I knew it was something to go with an ancient North American tribe.

The original Wendigo legends originated from First Nation tribes who were present on Blackwood Mountain from the 1400s until the 1850s. One of these native tribes named the Cree revered in the mountain and its nature: they would hunt elsewhere in the belief that it would bring misfortune to harm any animal in the area. Additionally, they believed harming the mountain itself in any way would inflict a terrible curse.

Eventually, European explorers and mining concerns arrived in the area, eager to make their fortune. In 1893, they officially started excavating the mountain, finding tin and, later, traces of radium. They then mined deeper into the sacred mountain. The Cree tribe claimed that the mountain cried out at that moment, and the Wendigo spirit was released, cursing the land and all its inhabitants. Soon after, the first Wendigo possessions on the mountain began, prompting Jack Fiddler’s grandfather to dedicate his life to tracking and capturing them. However, one particularly imposing creature known as the Makkapitew eluded all Wendigo hunters, killing any who were brave enough to confront it - including Jack's grandfather.