As we saw on reddit:
"[...]The Dark Knight will be using his MP to draw the power of darkness. But it’s not just a simple using of your MP to execute your actions but you draw from that power of darkness and you can either use it for an attack or use it to boost your defense and you’ll be strategizing on maintaining that power of darkness and managing your MP so it’s a very unique style of play in the tank role." - Yoshi P.

I think that power of darkness will be DKN's 'tank stance'. Some actions will only be usable under it, but it will drain his mp so he won't be able to be full power all the time...