A Pit Fiend (For DnD).

In DnD, there are devils and demons - both of them are the native denizens of the lower planes, both are incredibly powerful, and both of them generally are kept down below. Demons live on the Infinite Layers of the Abyss, and are raveningly mad, cruel, and full of destructive rage. Devils live in the Nine Hells of Baator, and are a plotting, scheming sort, hungering for power - they follow a strict hierarchy, and always keep their word, but the hierarchy is flexible if you can use plots and power-plays to increase your standing, and while devils always keep to their word... well, the literal meaning of their words can be surprisingly flexible. (The obvious moralizing is that making a deal with a devil is always going to be in the devil's favor as much as possible, any you only thought you were getting a good deal.)

But here's an interesting little difference. Demons are evil in nature, because they want to cause death, agony, perversion, and destruction. However, devils are evil in practice because they seek power by lawyering death, agony, perversion, and destruction.

Each and every devil desires to some day rise to the top, beat down the demons and angels and take control all of existence. Demons, on the other hand, are incapable of ruling, and therefore do not desire that sort of power (instead, they are power).

This leaves a bit of a loop-hole, and loop-holes make for interesting possibilities, and interesting possibilities make for characters. What if a power-hungry devil decides that love and kindness always seem to win against cruelty and enslavement, and decides that the practice of seeking power is more effective by building bonds of trust, admiration, and love, rather than bonds of fear, intimidation, and more fear.

Which produces a power-hungry devil that does good for selfish reasons.

I envision that a civilization in a DnD world is ruled by a benevolent-acting devil (Shalamshi) who provides free healing, devilish sorcery truly with no strings attached, and is understanding about deals... and basically is turning his civilization into a small utopia filled with people who truly, utterly think that the mystery figure behind it all is a good person. Armies of fanatics for self-defense, and an offense of conversion. Hopefully expanding until the whole material realm willingly accepts Shalamshi's rulership.

So for the Player Characters (PCs) who realize what is going on... what do you do? Do you kill the power-hungry devil, and make the world a worse place? Does intention outweigh action? Or do they fall in line, defending and expanding Shalamshi's no-strings-attached utopia? If they believe that Shalamshi will someday betray his vision, and turn his utopia into a dystopia? If so... when do you stop the goodness before it turns to evil? Should you stop it while things are good, since by the time things go bad, there will be nothing that can be done?

PCs have power in DnD - partly because the PCs have unlimited growth potential, but also plot armor in that they generally can win against anything they fight. However, with power comes responsibility - potentially responsibility for civilizations or even realms of existence. DnD therefore poses an opportunity to ask players, "You can do what you want, but what do you want to do?"

In my opinion, DnD isn't about players trying to win, but players trying to figure out what constitutes winning. Like what to do about an evil who does only good.