Lava courses through veins. Your skin is hot to the touch and anyone who comes in contact with more than a few drops of your blood takes 1d4 fire damage.

Since your natural form would scare the living crap out of most people, the fiend who sent you back granted you the ability to make minor changes to your appearence. You gain the ability to change your appearance after a long rest. This can only change basic body and facial features, and cannot change height, weight or add extreme physical features. (For example, you can't grow horns)

You don't have organs like other living creatures. The only thing within you is fire. Anything that you consume is instantly incinerated. The only way you gain sustenance is through consuming non-magical fire.

Your Constitution score increases by 1, and your Strength score increases by 2.

Hellspawn exist on their plane for as long as their fiendish creators allow. They are practically immune to the effects of aging, and generally don't keep track of it at all.

Hellspawn might not have an innate tendency toward evil, but many of them end up there. Evil or not, an independent nature inclines many Hellspawn toward a chaotic alignment. 

Hellspawn are about the same size and build as humans. Your size is Medium.

Your base walking speed is 30 feet.

Thanks to your infernal heritage, you have superior vision in dark and dim conditions. You can see in dim light within 60 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 red.

Your body was forged in the fires of hell. Any flame that is less powerful cannot hurt you.

You can speak, read, and write Common and Infernal, as well as one language you spoke in your past life.