One of my players would like to play a devil. His backstory is that his character was a arch devil in the 9 hells, but was kicked out. I've decided to help him create the race, and these are the racial traits I have so far:
Infernal Resistance:Gain Resistance to fire damage
The Sword of the Archdevil: You can automatically light any single weapon you are proficient with on fire. This fire damage does 1d4 damage. If you hit someone, they are lit on fire, and must make a DC 10 dex save to get it off.
Infernal Weakness: You are vulnerable to radiant damage and cold damage
Shape Change: What I'm having trouble with. The player wants to be able to disguise his character into human form, and gain the devils stats when he changes back. How do I do that?
Thanks! :)
A lot of the monsters that shapechange keep all their stats for the most part, so it's mostly cosmetic. Sometimes they gain or change their natural attacks or have a minor change to AC or speed, etc.
Otherwise, you can use the polymorph rules, which essentially creates a "shell" of bonus hit points, and you revert to your normal form (and hit points) when the shell's hit points reach 0.
The polymorph rules might be VERY powerful for a racial ability. A disguise self-like power might be more appropriate.
I would suggest looking at the tiefling and swapping out the magic abilities. You can use radiant damage vulnerability as a minor balancing factor, but cold damage vulnerability might be a bit much.
EDIT:
For Sword of the Archfiend, maybe have it cause additional fire damage equal to your proficiency bonus. That way, it scales with level, does not multiply on crits, and reduces the number of dice being rolled for ease and speed of play.
How about replacing shapechange with a feature that depends on what type of devil the character is? For example if the character is a horned devil or pit fiend, they might gain wings, while as a Ice devil, the character might gain ray of frost or another cantrip?
Wings vs. Ray of Frost are disparate in power. Radiant damage is also very rare among monsters.
I'd honestly suggest using Tiefling as is, and flavor class abilities he gets as unlocking his greater devil powers.
Maybe swap out his racial spells for Dancing Lights/Burning Hands/Alter Self, per one of the setups the variant tiefling might get.