Thermal Sunder

As an action, you sap the heat from the area around you, leaving the vicinity in an icy void. Each creature in a 10 foot radius around you must make a Constitution Saving Throw or take 4d8 cold damage on a failed save, or half as much on a successful one. For each target that made it's save, gain a single charge of Frostfire, and gain two charges for every target that failed it's save.

You may choose to expel the stolen heat immediately after the cold snap during any of your turns afterwards for the next minute as a bonus action. When expelled, the heat blasts out in a 15 foot cone in a direction originating from your location. Each creature in the cone must make a Dexterity Saving Throw or take fire damage on a failed save, or half as much on a successful one. The amount of damage dealt is equal to the amount of Frostfire charges held as d8's. Four Frostfire charges is 4d8, seven charges is 7d8, etc...

When holding the heat in, losing concentration of the spell or having a minute pass since absorbing the heat causes the built up energy to dissipate, ending the spell.

At Higher Levels: When you cast this spell using a spell slot of 6th level or higher, the cold damage increases by 2d8 and the fire damage increases by an extra Frostfire charge for each slot level above 5th.