The Infernal Legacy grants a Tiefling the ability to cast Hellsih Rebuke as a 2nd level spell and the handbook says that you use Charisma for the spellcasting ability of these spells. So if my Ranger Tiefling has a Charisma score of 14 would the Spell Save DC for Hellish Rebuke's damage be 10 (8+Charisma Mod of 2) or would I add the proficiency bonus to it as well?