Cryomancer

You grew to be cold, frozen and frostridden, your body is chilled to the ice age

as you grew cold and hoar frosted. you found yourself ready for anything

Level 1: Heart of Ice

At 1st level, whenever you start casting a spell of 1st level or higher that deals cold damage, icey magic erupts from you. This eruption causes creatures of your choice that you can see within 10 feet of you to take ice damage equal to half your sorcerer level (minimum of 1). 

Level 6: Frost in The Veins

At 6th level, you gain resistance to cold damage. In addition, spells you cast ignore resistance to cold damage.

Level 14: Cryomancer's Frost Fury

Starting at 14th level, when you are hit by a melee attack, you can use your reaction to deal cold damage to the attacker. The damage equals your sorcerer level, and ignores resistance to cold damage.

Level 18: Frosty Soul

At 18th level, you gain immunity to cold damage. In addition, any spell or effect you create ignores resistance to cold damage and treats immunity to cold damage as resistance to cold damage.