When playing as Frodo in Rivendell, get to the room where you get sting. You will see two chests. One will have an Elvish tome. Give it to Gandalf and he will say something. Then, talk to Elrond. When playing Gandalf after fighting the Wargs, use the tome. You will get a new spell "Sleep". This will put enemies to sleep for a short period of time. Note: Make sure you give Gandalf the tome when playing as Frodo or he will not have it when you are playing as him.