There are plenty of sources to tell you more about pit fiends, in particular F. Wesley Schneider’s Princes of Darkness for Pathfinder and Tyrants of the Nine Hells by Robin D. Laws and Robert J. Schwalb for 3.5. And that doesn’t even touch the years of Manuals of the Planes and Planescape supplements.
Instead, I want to talk to you about the psychology of pit fiends. Imagine this: After being incarnated as a devil—sometimes as low as a base lemure or even one of the damned, you have risen through the ranks of devilkind. Along the way you have had to endure and overcome (not to mention simply survive) gruesome tortures, deadly environments, appalling overseers, backstabbing peers, duplicitous underlings, and perhaps even the summons of impudent mortal fleshlings. Even the act of promotion is a torment—an excruciating metamorphosis that changes your very morphology, character, and outlook, and which takes you out of the action (and thus out of influence and power) for up to centuries.
Yet finally, you’ve made it. You are a pit fiend. You are on top. A master. The master.
Only. Only you’re not.
Your underlings want to stand where you do, just as you once envied and aimed for your superiors. So watch your back. And you still must follow orders and earn influence—from more experienced pit fiend generals and ambassadors, the various infernal dukes (and Whore Queens and the malebranche, in Golarion’s Hell), and of course the Lords of the Nine themselves, some of whom now know you by name. So don’t screw up.
You have reached the pinnacle, only to discover another mountain. You’re like the valedictorian who got into an Ivy League (or better) school only to find a college of people smarter than her. You’re the newly minted CEO now being judged by far more successful peers, the NYSE, and The Wall Street Journal. You have spent centuries, even millennia, fighting to discover that the princess is definitely in another castle, and that castle belongs to a god: Asmodeus.
And there is no escaping the game. Solars and ghaele paragons would hunt you down. Gods would spurn you. Hell itself—the actual plane—would not let you leave, literally forming rocky fists to hold you down and strip you of your status. Not that you would leave if you could. Outsiders can change their habits, tactics, and allegiances, but almost never their alignments and desires.
You are truly damned. And you hate or despise literally everything in existence.
Now imagine some mortal tries to summon you, or interfere with your carefully laid, piano-string-taut plans that you have spent centuries tuning.
Damn right you would come down on him like a ton of Dis’s cobblestones.
From his guise as a humble vizier, a pit fiend has guided the Blessed Empire of Quirinus for close to three hundred years—founding five cities in the process. When plague breaks out in Karis, a tsunami swallows Port Royale, and locusts beset Memsin, a disgraced centaur sage posits that these three destroyed cities form three points on a pentagram. In a rare overreaction, the pit fiend tries to have the centaur silenced, tipping off adventurers that he may be on to something. And there are two cities as yet unmolested…
A pit fiend is among the guests at a banquet for the Titan of Memory, as are a solar, an intelligent shoggoth, a half-dragon nephilim, several oni, an aeon, and a night hag transmuter (who also caters). An axiomite has brought an adventuring party as bodyguards, and cannot explain why the pit fiend welcomes them as old friends.
The pit fiend Legulos has attained the station of duke. As part of his transformation, he trades hellfire for soulice, grows two more heads (one to watch the first and one to spy on the second), and builds a palace of ice. His portfolio is false inquisitions, and he has a special torment planned for a mortal inquisitor who eluded him in the past.
—Pathfinder Bestiary 80–81