You Unlock the "Special Chapter" after you beat the Story Mode. You have to Face Akuma as Devil Jin.
Akuma can be hard to Defeat. However, it's much easier if you know how to Drop Kick.
Press
+
to Drop Kick. This makes it easy to defeat Akuma. Each drop kick takes about 15% Damage.
**IMPORTANT**
Once Akuma's Life is Depleated, he will Trigger his Rage Art. This will One-Hit-Kill you unless you execute your Rage Art to Counter it. To do this, Simply press
to execute your Rage art. (Spam it once Akuma's Rage Art Cut-Scene starts)
Video Guide: