It’s only used once in The Hobbit, compare with less-formulaic-magic-connotation “enchant”.  Per OED:

The use of a formula of words spoken or chanted to produce a magical effect; the utterance of a spell or charm; more widely, the use of magical ceremonies or arts; magic, sorcery, enchantment.

• 02.111 while Gandalf tried various incantations.

“incantation, n.” OED Online. Oxford University Press, June 2015. Web. 25 July 2015.