What does Harry do to defeat the Nightmare, in Chapter 21?

Harry picks up Amoracchius because Michael is carrying his wife out of the graveyard. Lea shows up and demands that Harry go with her; Harry becomes enraged at her demand, because he hasn't killed the Nightmare yet, so he swings Amoracchius at her. The sword flies out of his hand, and because he used it for a purpose it was not intended for, Lea is able to pick it up and return to Nevernever with it.