A History of Religious Ideas, the third volume because my library didn’t have the others, by Eliade.
[…]The sufferings of the “elect” bear a point by point resemblance with initiatory tortures. Just as, in rites of puberty, the novice is killed by demon-“masters of initiation”, so the future shaman sees himself cut up and morselled out by the “demons of the illness”. In his sickness, he experiences his ritual death in the form of a descent into Hell. He witnesses, in his dream, his own dismemberment. He sees the demons cut off his head and scratch out his eyes. According to the Yakuts, the spirits carry the future shaman to Hell and imprison him there for three years in a house. It is there that he undergoes his initiation: the spirits cut off his head, which they put aside (because the novice must look on with his own eyes as he is torn apart), and they cut him into little pieces, which they then distribute to the spirits of diverse illnesses. It is only by experiencing this condition that the future shaman will obtain the power of healing. His bones are then covered over again with new flesh, and in certain cases he is also supplied with new blood. Other shamans tell that during their initiatory illness, the ancestral shamans pierce them with arrows, cut their flesh, and pull out their bones in order to clean them; or else they open up their stomach, eat their flesh and drink their blood; or cook their body and forge on their head with the use of an anvil. During this time, they lie unconscious, nearly inanimate, for three to nine days in the yurt or a solitary place. Some seem even to have stopped breathing and have nearly been buried. Finally, they are resuscitated, but with an entirely renewed body, and with the gift of the shaman.
According to Eliade this is the general tradition of both Siberians and Finno-Ugric people like, say, the Sami. Now I understand why Scandinavian death metal is so hardcore. :P