Hellspawn - Fiendish Resistance

At 14th level, your body begins to go under irreversible physical changes due to the hellish form you don.

You gain resistance to fire and poison damage, as well as advantage saving throws against being poisoned.
While you are in your Demonic Form, whenever you are affected by a spell or magical effect that requires a saving throw, you can use a reaction to give yourself advantage on that saving throw.

You also are permanently altered even while you aren't in your Demonic Form, gaining a forked tongue, curled horns, or some other appearance that only a fiend may have.