The fuaran is magic book capable of transforming humans into Others. When the book is used by an Other every human in his field of vision will be transformed into an Other.
The book works by lowering the magic temperature of humans. It can be used repeatedly on the same person. With each use the Other targeted becomes more powerfull.
The fuaran is believed to have been destroyed at the end of "Twilight Watch".It was recreated in the first book of new trilogy.[SOURCE??]