I had great fun adapting/creating some race-classes previously for use in Whitebox, you can see my prevous post here. I’m also a huge fan of the Tiefling in 5E so it seemed natural to make a race class for the Tiefling but it took me a while to consider what niche it may fill in relation to existing classes. Then I remembered reading in an old Planescape product that Tieflings were often the result of unions with Succubi, looking at the 5E Compendium version of Tieflings their main score was charisma, so I hit on the idea of making a class with an ability allowing them to adjust the initial reactions of NPCs when first encountered.
Update 1st May 2018 07:55 – It was suggested to me (I’m sorry but I forget the name of the person who suggested it) that I change the fire resistance so that–instead of being a variable amount based on level–that it simply halves fire damage, I think this is a great idea and makes for less admin, so I’ve altered the class. The link above now points to the altered version.