Hellboy has many distinguishable features, from his gruff nature and quick wit to his filed-down horns and long red tail. Perhaps none of these, however, are as recognizable as his large stone fist, with its rocky make-up and four thick digits perched on the end of his relatively normal right arm. As it turns out, Hellboy wasn't born with this powerful appendage, so where exactly did his Right Hand of Doom come from?
Simply put, it's not really his. It actually belonged to a being called Anum, who was part of a race of spirits called the Watchers, created by God to watch over the world. When on Earth, Anum used his hand to create the Ogdru Jahad, who are also known as the Dragons of Revelations and are hellbent on the destruction of everything and everyone. From these beasts spawned Ogdru Hem, abominations so despised by the Watchers that they went to war with both them and their creators. After defeating the Ogdru Hem, Anum once again used his right hand on the Ogdru Jahad, this time encasing them in a crystal prison. Now distrustful of Anum, the other Watchers destroyed him, saving only his right hand from evisceration.
God then gave some of the Watchers more physical forms as they took up residence on Earth, becoming the first race of men known as the Golden People of Hyperborea. They believed that the Right Hand of Anum was an extremely holy object, attaching it to a giant statue within their capital city of Gorinium. When the people began worshiping another God, the statue came to life, killing the people of Gorinium and staining itself red with their blood. The statue then threw itself over a cliff and shattered, Only the Hand survived.
The Hand was recovered by the demon Azzael, one of the Dukes of Hell. After impregnating a woman, Azzael took her to Hell where she died giving birth to the Hellboy. Azzael removes the child's arm and grafts the Hand of Anum onto him, then releases him into the world. When Hellboy is summoned later in life by Grigori Rasputin, an agent of the Ogdru Jahad, he discovers that the Hand can release and command the imprisoned Dragons of Revelations, and would therefore complete its true purpose to bring about the apocalypse, hence the name "The Right Hand Of Doom." It never explicitly says within the comic series how it would do this, and Hellboy never finds out as he refuses to end the world.
It is believed that the Hand does not require Hellboy and would be able to fulfill its mission with another host or even completely autonomously, which is why he has gone to great lengths to keep it -- believing it to be much safer with him. While it isn't a part of his original body, he is able to operate it as if it were his own hand, although it has only four digits instead of five. It's effectively invulnerable and feels no pain, and Hellboy wields it almost like a sledgehammer, using its size and weight to add to his own immense physical strength. It isn't ideal when using weapons or tools, but he himself has stated he much prefers using brute force and his fighting skills, meaning the fist becomes a great asset.
While the true function of it may be to bring about the end of the world, Hellboy has used The Right Hand of Doom to save the world from mystical threats on countless occasions, showing his true defiance against his demon heritage.