Your Dexterity score increases by 1, and your Charisma score increases by 2.

Tieflings mature at the same rate as humans but live a few years longer.

Imps are tiny

Speed. Your walking speed is 20 feet.

Flight. You have a flying speed of 30 feet.

Thanks to your infernal heritage, you have superior vision in dark and dim conditions. You can see in dim light within 120 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

You have resistance to fire and cold damage.

You can cast shapechange as an action, and have a natural weapon.

Shapechanger. As an action you can polymorph into a rat, raven, spider, or back into your true form. Your statistics are the same in each form, except for the speeds of each form. Any equipment you are wearing or carrying isn't transformed. You revert to your true form when you die.

Natural weapon: 

Sting. You have a natural weapon in the form of a venomous barbed tail which you can use to make an unarmed strike. If you hit with it, you deal piercing damage equal to 1d4 + your Strength modifier and the target must make a Constitution saving throw (DC 8 + your proficiency bonus + your Constitution modifier), taking an additional 1d6 poison damage on a failed save, or half as much damage on a successful one.

This damage increases to 2d6 at 5th level, 3d6 at 11th level, and 4d6 at 17th level. You can use Dexterity instead of Strength for the attack and damage rolIs of your unarmed strike. Your unarmed strike counts as a weapon with the Finesse property for all effects which require it, such as Sneak Attack.

You can speak, read, and write Common and Infernal.