McTaggart is Michael's loyal servant. He took the book to prevent his master from turning evil.
Michael had him immortalised so he could torture him in his dungeon for eleven hundred years. He then had to eternally be a servant until the book was found. Despite this curse, McTaggart remained loyal to Michael Scot throughout the 1100 years of immortality and was at his side when he died.
McTaggart was transformed into a weasel and placed in a cage by Toledo, until freed by Edwin and Ailsa. Despite not being trained in magic he managed to "learn the odd spell" in his years in Scot's service. McTaggart died triumphantly in battle against the Dawn Queen and fell to his demise, disappearing into a river - despite the fact he was immortal because the book still hadn't been found!? His death was mourned by his friends, and also even by Toledo (in the form of Aurora Dexter). His ghost later came back to give Marnie some advice.