Random thoughts on someone who just became a Pit Fiend (but is not actually evil himself):
1) Is he turning into a Pit Fiend or a Native Pit Fiend? If the former, that means that, if killed while in Pit Fiend form, his soul, which forms one unit with his body (SRD), immediately departs, and can't be restored except by a Limited Wish, Miracle, Wish, or True Resurrection.
2) He's affected by stuff as though he has the Lawful and Evil subtypes. The Lawful can be just as important. A good ol' demon is going to be slinging some chaos magic his way, most likely. A Holy Word by his buddy the Good Cleric is gonna be unpleasant for him.
3) That SR is a double-edged sword. As a bad guy, my biggest priority is to do something nasty and spell-like to this pit fiend sorcerer, because as soon as I've got something nasty going on with him, he's got SR32 against his buddies' attempts to undo it -- ie, I'll be casting a lot of slow spells, because hasting him back to normal requires beating SR32. The only time SR doesn't apply is when I'm casting spells on myself or when I voluntarily take a standard action (if those still exist in 3.5) to do so, meaning that my SR is then down until the beginning of the following round.
I'm not sure from the reading whether you'd need to lower SR to benefit from a friend's attempt to teleport you to safety, but it's worth looking into. In my high-level games, everybody needed to be targetted by spells by friends sometimes -- a Heal spell, for instance
So basically, as a bad guy, my defense against this guy would be to get out of harm's way and fire off a boatload of low-level attack spells -- little stuff that only requires a touch attack to hit, or that simply goes to SR and saves. If I'm a high-level bad guy fighting this person, I've got a lot of Slow, Polymorph, Enervation, Blindness, Touch of Idiocy, and so forth. Going toe to toe with this guy is a horrible idea (although it does turn him into a less-than-totally-effective tank instead of a massively buffed 18th-level spellcaster, so maybe I have a golem minion go grapple him or something) -- instead, I harry him, fire off Dispels along with those nasty debilitating spells that, at this level, I have in bunches. Most bad guys don't get to use these spells, because the DM goes for the big money spell right off the bat. In this case, the big money spell isn't likely to work on this guy. You need to break him down first, frustrate him.
But as other folks said -- this is the most powerful spell he'll ever know (or one of the few of this level that he'll get). He deserves to shine with it for awhile. Just make sure that your module doesn't give him enough rest opportunities that there's no reason for him to not be fiended out every time a fight rolls around. And have him show you on his character sheet how many extremely expensive jade circlets he's got with him before he sets out on the adventure.
Oh, and jade... that's a gemstone, right? Gemstones are crystal, right? So if somebody cast Shatter or Shout on you early in the combat, before you went all Fiendish... that would be bad?
Beyond that, look at the environment. There are a lot of environments where being 12 feet tall and weighing 800 pounds is not a great plan. A dwarven stronghold might have, you know, 7 foot ceilings -- the humans can handle things, although any overhand swings are likely to scrape the ceiling (-1 on attacks), but our buddy the Pit Fiend is in a bit of a predicament.
![:]](http://www.enworld.org/forum/images/smilies/devious.png "Devious :]")
