This achievement is self-explanatory except for that it appears to be glitched for some people (myself included). I didn't beat MK I, II, or III until after I had the december title update. Beating each of those granted me their respective achievements, but I did not unlock "The Chosen One."
I saw a piece of advice on x360a.org that recommended disconnecting from xbox live and playing the games again. I disconnected from Live, beat MK I again, and the achievement popped immediately after beating Shang Tsung. Hopefully this works for others.