The Demon Bear is the main antagonist of Wolf Brother, the first part of Michelle Paver's Chronicles of Ancient Darkness series, which is set 6000 years ago in prehistoric Europe during the Stone Age.
Before the start of Wolf Brother, the demon bear is created by Tenris, a Soul Eater who poses as a crippled wanderer from the Willow Clan in order to kill Torak's father (who happened to be Tenris' brother). He stays with the Red Deer Clan and builds a shelter where he invokes an elemental (the strongest type of demons) with the Fire Opal and imprisons it in the body of a bear cub Hord of the Raven Clan had hunted for him. Unfortunately, the bear is too strong and Tenris loses control over it. The bear manages to kill three people of the Red Deer Clan before they chase it away, and Tenris flees as well.
After its creation, the bear kills every living thing it encounters in the Forest, barely eating any of its victims (which even flies don't touch). Every time it kills somebody or something, its power grows. In the first chapter of Wolf Brother, it finds and kills Torak's father. Before dying, the former Wolf Mage makes his son to swear an oath to go north, find the Mountain of the World Spirit, and ask the World Spirit to help in destroying the bear. Torak has to reach the Mountain before the full moon, because then the red eye of the Great Auroch (the strongest demon to have ever existed), a celestial body of great evil is at its zenith and the bear becomes invincible. Torak soon finds an orphaned wolf cub, which helps him to find his way on his quest to destroy the demon.
After being captured by the Raven Clan, Torak discovers that there is a prophecy about the demon bear. It says that the Listener who "talks with silence and fights with air", will offer his heart's blood to the World Spirit that then kills the demon bear. One interpretation of this prophecy is that Torak must be sacrificed and his blood must be taken to the Mountain. Luckily, Torak and Wolf escape with help of a Raven girl named Renn, who believes that Torak must go to the Mountain himself. She also reveals him that as the Listener, he must find three crystallizations of the Nanuak (a flowing life-force that's in every living creature) to please the World Spirit, who is angry because Tenris violated an ancient pack that forbids one to create a creature that kills needlessly.
Unfortunately, the demon bear can scent the Nanuak and starts following Torak and his friends after they find the first Nanuak part. Later, the bear corners Renn and Wolf in a small cave (with Torak having ventured deeper to find the second Nanuak part) and tries to kill them. Suddenly, Wolf grabs the first part of the Nanuak from Renn, runs out of the cave and away among the trees with the bear in pursuit. Wolf successfully decoys the demon bear away and then returns back to Torak and Renn.
Off-screen, the bear faces off the Raven Clan, kills two of their men and injures Fin-Kedinn's leg, forcing the Raven chief to use crutches for the rest of his life.
Eventually, Torak gets all three parts of the Nanuak and climbs with Wolf to the Mountain of the World Spirit, followed by the bear demon. Suddenly, Torak is attacked by Hord who wants to take the Nanuak to the Mountain himself. Torak then realizes that the prophecy's "heart's blood" means Wolf. He sends Wolf to carry the Nanuak up to the Mountain. The World Spirit then hears their calling, after which the bear and Hord are engulfed by an avalanche and fall down the Mountain. The elemental is banished to afterlife, and the bear's own souls probably receive peace after being imprisoned in its body with the demon.