Exorcism in Christianity
Exorcism in Christianity is the practice of casting out demons from a person they are believed to have possessed. The person performing the exorcism, known as an exorcist, is often a member of the Christian Church, or an individual thought to be graced with special powers or skills. The exorcist may use prayers and religious material, such as set formulas, gestures, symbols, icons, amulets, etc. The exorcist often invokes God, Jesus and/or several different angels and archangels to intervene with the exorcism. A survey of Christian exorcists found that most exorcists believe that any mature Christian can perform an exorcism, not just members of clergy. Christian exorcists most commonly believe the authority given to them by the Father, Son, and Holy Spirit (the Trinity) is the source of their ability to cast out demons.
The Cairn on the Headland
Odin being in Howard's interpretation a demon, his banishment at the climax of the story is an effect an exorcism, and it includes many elements of an exorcism as depicted in Christian tradition. Holding out a crucifix is commonly mentioned as an effective method of confronting a demon. According to Christian theology - specifically, Catholic theology - one need not be a clergyman in order to perform an exorcism. However, if one is not especially holy oneself, the chances of overcoming the demon are much increased if one has a holy relic connected to a venerated Saint, thus being able to draw on that Saint's holiness. In all that, O'Brien follows on the well-established rules for carrying out an exorcism - and it works.