Question: I am designing a campaign and I have a couple of evil power blocs vying for control. I would really like to have one of them directly interacting with the party and I think I really want it to be a devil because I love the idea of the lawful evil tricking them while still being true to its word.

So my question would it be possible to disguise a devil as a human\* using canon mechanics? I know I could just house rule that the devil has the spell polymorph or something but I was wondering if there was a way to do it within the rules as written for 5e?

Now, this doesn't have to be an undetectable disguise. I am fine with the players discovering he is a devil in disguise if they look hard enough. Also, the type of Devil can change if needed, though I would prefer to have a more prestigious Devil that is calling the shots.

I have read the entire section on Devils in the Monster Manual and there isn't anything obvious. I was thinking of an Erinyes with a Hat of Disguise Self but I don't think Disguise Self would be able to conceal the wings, so they would have to pretend to be an Angel which I wouldn't really consider "normal" to be interacting with.

As for making a custom monster according to the DMG I would consider that akin to house ruling it. Which is definitely something I can fall back to, but I was trying to think of something more direct from the rules.

\*Or anything else that would normally interact with a party around a town.

Answer: 
Hide in plain sight. There are two good options for this:

**Erinyes** Based on the picture on MM 73, these creatures are mostly humanoid, but have wings. The horns on the helmet may or may not cover any natural horns; it's unclear. Either way, you could throw on a cloak to hide your wings, then claim to be a Tiefling. Depending on the campaign world, there may be any number of them around, trying to redeem their unsavory lineage.

**Asmodeus** If you'd like to go straight to the top, you could send in the lord of the lowest of the Nine Hells. He's described as "a handsome, bearded humanoid with small horns protruding from his forehead, piercing red eyes, and flowing robes." He can also assume other forms. He should easily pass as a Tiefling.
Why not try using the disguise of an entirely different person? 

In the MM, all the stronger devils seem to have telepathy of 120ft. They could use their telepathy in order to communicate through another person that is in service to the devil. I think it would make sense that a higher more prestigious devil would employ servants to do dirty work.

This wouldn't be entirely undetectable. The devil would have to remain 120ft close to tell the Servant what to say or maybe even closer to hear the PCs(depending if you consider telepathic communication a 2-way street. I could not find a straight answer on this.) A wise PC might could notice the words of the Servant are not the Servant's own, maybe a hesitation in the Servant's voice which would require investigation, or sense they are being spied on a powerful being.