Okay, I'll offer up a few comments.
First, I think the flavor and focus of the class is fine. The special ability selection is, for the most part, appropriate and reasonable.
Technical points - you need to specify the hit die type (d8, d10?) and number of skill points per level (presumably 2+Int). Probably wouldn't hurt to clarify which abilities are supernatural, extraordinary, or spell-like. This was done for a few of them, but all should be clarified.
Now, after reading through it, my first thought was that it is quite overbalanced. The main reason for this is that you've given the class a full fighter's BAB progression, a full cleric's spell progression, plus a very healthy dose of special abilities at every level. For starters, because the flavor of this class and the abilities focus much more on the martial aspect of combating fiends than the magical, I'd suggest reducing the spell progression to +1 every other level.
For the special abilities, I only have a few smaller suggestions. First, I would consider changing the Suppress Summoning so that it allows the fiend to make a Will save to perform the summoning...say with DC=10+Protectorate level+Cha bonus. This ability could then become available at an earlier class level.
Second, I think the Improved Smite Fiend is a bit much. The class already has the powerful Smite ability plus they've got +3 or +4 bonus to-hit and dmg from the Fiend Hunter ability by the time this would become available. If anything, I might just consider giving them the feat Extra Smiting (allowing them to use it an additional time per day).
Third, I'd drop the spell resistance altogether. You've already given them substantial bonuses to Saving Throws with the Spurn Fiend's Touch ability as well as the Immune to Mind Affects.
Even should you implement all the changes I suggested, it may still be a bit on the powerful side. I'd suggest going with a d8 hit die. If you really want to keep all of the special abilities, I'd still suggest dropping spell progression to every other level as well as decreasing the BAB to that of a cleric.
All-in-all, though, it's a good starting point for a reasonable prestige class. Good luck with it.
