Board Thread:General Mod Discussion/@comment-98.184.77.53-20150314150739/@comment-25869554-20150316211618

I think that Merry's ancient sword from the Barrow-downs (a blade of Westernesse) broke the spell that kept the Witch-King's body together. Thus, Eowyn was able to kill him.

The Tolkien Gateway's article, Daggers of Westernesse should explain.