Fenrir is a vast god like wolf, his only role is to destroy the earth, his vast size gives him the power to devour anything and has the strength to even defeat gods.
Fenrir originates from Norse Folklore the story has been around since the 13th Century. Fenrir is a godlike wolf and is the offspring of Loki and the King of the Wolves, he is foretold to be the one to devour Midgard (Earth) and defeat Odin.
Because of his vast size he is only threatened by gods and other mythical creatures because humans cannot harm him at all, also because of his size he gain superior strength allowing him to actually battle with some of the most powerful Norse gods. Fenrir is destined to devour the earth, just this sentence shows how feared he actually is and the strength he holds.