A Nice Puzzle

I've decided to start a contest. I'll give you a chess problem, and you have to name the "key"- the first move that leads to checkmate no matter what black plays. no cheating- if you've seen this before, no answering. and no using anything except a board & pieces-no internet.

The winner will recieve a prize! :) i will post a comment when the key move is found. no winners yet. 

EDIT: Contest #1 goes to diomed1!