MMA meets Pathfinder.
I have to say that I am neutral overall about this one. I like the setting tie in and potential for Hellknight usage but I wonder if it is too setting specific.
Would I want to play this character? Unsure.
This being said however it is sound mechanically and I could see it being played.
Despite my neutrality on it I would have to recommend it for advancement.
Impaler (Fighter)
NOTE: I read all archetypes before voting, but didn't get all comments posted before voting. I'm now finishing my comments.
Campaign specific, but overlooking that...
The concept of a fighter who focuses on wearing spiked armor. This is a good concept.
Unfortunately, I think you missed an important point.
For Retaliatory Strike and Grasping Barbs, how many times per round can he do this?
Also, for Grasping Barbs, does the Impaler HAVE to grapple the attacker? I could see someone using that against him. "Quick throw the kobold at the Impaler. AHA! Now he's grappling the Kobld, get him!!!"
Just my thoughts,
Ken