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.
A hat of disguise could easily allow an erinyes pass off as an aarakocra or winged elf, either of which would stand out in a crowd, but wouldn't be barred entrance to a city.