Draconic Critical

You can channel the unfettered wrath of powerful dragons into your blows.

You get a critical hit on a roll of an 18, 19, or 20. Additionally, whenever you make a critical hit, you can have your attack deal an additional 2d10 of a damage type of your choice from the following: acid, cold, fire, poison, lightning, necrotic, radiant.

When you choose this feat at 14th level or higher, the damage increases to 4d10 and if the target is reduced to 0 hit points by this damage, they must succeed on a Constitution saving throw (DC 12 + your proficiency bonus + your Wisdom, Intelligence, or Charisma modifier) or die instantly.