Sorcerous Origin: Cryomancer

A reflavored port of the Pyromancer subclass from the Plane Shift: Kaladesh compendium for Magic: The Gathering, which is listed as being compatible with Dungeons and Dragons 5th Edition.

Heart of Ice

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

Ice in the Veins

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

Cryomancer's 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 sorceror level, and ignores resistance to cold damage.

Icy 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.