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: 
Consider Titivilus
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Mordenkainen’s Tome of Foes includes a number of devils that you can draw on for your campaign. One of these is [Titivilus](https://www.dndbeyond.com/monsters/titivilus) who can innately cast [*Alter Self*](https://www.dndbeyond.com/spells/alter-self) and [*Nondetection*](https://www.dndbeyond.com/spells/nondetection) at will. *Alter Self* would allow him to easily pass as a human and *Nondetection* would help prevent his true nature from being discovered.

Other relevant spells include *Modify Memory*, *Major Image*, and *Mislead*.

Titivilus is the second in command to Dispater making him important enough to count as prestigious while still being lowly enough that he would personally manage his schemes on earth.
When you say 'devil', would *any* devil do? If so, and given your "anything else that would normally interact with a party around a town" then - for a given definition of "interact" an imp works. It's technically a devil and can shapechange at will into a spider, rat or raven, all of which can be encountered in a town without raising suspicion. It retains its statistics in the new form which means it can speak Common and Infernal. You could either have it claim to be an Animal Messenger of a party who wishes to remain anonymous, or have it perch on the shoulder of a zombie (or corpse, or just someone paralytically drunk or intimidated or bribed into going along) disguised to look like a well-wrapped wizard. Nobody is going to assume the voice is coming from the spider.