Blessed by Tiamat

Tiamat has blessed you with her power, perhaps because you impressed her, or because you are her descendant. Your breath weapon is transformed into a maelstrom of elemental energy, and you become much better at channeling it. You gain the following benefits:

• When you hit a creature with your breath weapon, when calculating damage for vulnerability, resistance, and immunity, it always takes the path of least resistance among Fire, Lightning, Poison, Acid, and Cold damage. If a creature is vulnerable to any of these damage types, it deals damage as if that creature was vulnerable to your breath weapon type. It must be resistant or immune to all of these types at once in order to gain the benefits of its resistance or immunity.
  
  
• You may use your breath weapon a number of times equal to your proficiency bonus, recharging on a short rest.
• Your breath weapon counts as each of fire, lightning, poison, acid, and cold damage for the purposes of features such as elemental adept, elemental affinity, or destructive wrath. 
• You may use a bonus action to expend a hit die. If you do, you gain resistance to one of the elemental types listed above until your next long rest.