Cyromancy

Your innate magic manifests in frost. You can freeze and create icicles at a snap of a finger, using ice magic to freeze and slow creatures or sharp icicles and tough snow to pierce or bludgeon your foes. Cyromancers emanate an aura of cold that chills those around them, despite the fact that if anything, most of the time, most areas feel too hot for them. 

Winter Winds

At 3rd level as you channel your icy magic, you can at the same time chill the area around you. Immediately after you cast a spell of 1st level or higher, you can cause chilling winds to swirl around you, creating a 15f radius centered on you of difficult terrain to creatures attempting to move through the winds. This lasts until the end of your next turn. This does not affect you. These winds will freeze any non-magical water that isn't being carried.

At 6th level, creatures you designate can ignore the difficult terrain around you. These creatures gain resistance to fire damage while within the aura.

At 18th level, the aura increases to 20f. 

Heart of Frost

Also at 3rd level,  the cold magic coursing through you heightens your resilience to any similar magic. You gain resistance to cold damage and while you have your winter winds feature active, you have resistance to fire damage. In addition in the case your campaign is using survival mechanics. you can ignore the effects of low temperature. 

Contained Chill

At 6th level spells you cast can ignore resistance to cold damage. In addition, you can protect creatures from your cold spells, creatures you designate automatically succeed the saving throw against taking cold damage. If you have careful spell metamagic, you can protect double the creatures you could otherwise from cold damage. 

Icy Tomb

At 14th level as reaction to taking damage, you can condense your cold magic to make a final stand. You can take this reaction after seeing the damage rolls, but before any effects would occur. You surround yourself in an icicle, the icicle has hit points equal to 25 + 2x your sorcerer level and an ac of 10 + your spell attack modifier. While in this state, the icicle takes damage for you, and you cannot be targeted, you are considered incapacitated. The condition ends once the icicle loses all its health, you dismiss it or after 1 minute, at which point it melts into a puddle. Then you must long rest to use it again, unless you choose to regain the use by spending 5 sorcerery points as an action. 

Absolute Cold

At 18th level you're beginning to reach the peak of Cyromancy. You gain immunity to cold damage and spells you cast can treat immunity to cold damage as resistance instead. 

In addition you can chill an area to a point of almost absolute zero. As an action choose a 10f cube, creatures within that area must make a Constitution saving throw. On a failure they take 5d12 + 50 cold damage or half on a success. Regardless of success or failure, those creatures have their movement reduced to 0f until the start of your next turn, and gain vulnerability to bludgeon damage, if they are not resistant or immune to magical bludgeoning.