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.