Merry doesn’t get enough credit for stabbing the Witch-King of Angmar in the back of the knee with his blade from the Barrow-downs, which bears a special enchantment against the forces of Angmar. Without this act of courage, Éowyn would not have been able to deliver the final blow. 
Discussion
But I love that it was a woman to put him down.
If you believe that the characters inside the story actually know what's going on...
Merry's blade dissolved into smoke.
Eowyn's blade broke, and could presumably have been reforged just like Narsil was.
"All blades perish that pierce that accursed king" -Aragorn (quote from memory, didn't look it up)