Null Fighter

When the very ground you walk on is sewn with arcane corruption, and the air in once populated ruins is laced with volatile magics, you learn quickly how to avoid such things, or die trying. Null Fighters represent those who not only learnt to avoid, but to harness those chaotic powers, channelling vast amounts of wild magic into blades and trinkets, containing it for later usage, or dispelling it.

Gameplay-wise, they focus on nullifying and countering Area of Effect spells as well as all forms of difficult terrain. Manoeuvring around these things also benefits them with powerful attacks and greater movement speeds, allowing them to close in on their spell-slinging foes.

Level 3: 3rd Level: Hardened

Your resolve in the wastelands of this new world has strengthened. When you use Second Wind, you can add half you Constitution score to the roll, when determining your hit points recovered. Upon doing so, you gain resistance to Psychic and Force damage until the start of your next turn.

Level 3: 3rd Level: Cut-throat

Immediately after you successfully save against a spell, you can move up to half your Speed without provoking Opportunity Attacks.

Level 7: 7th Level: Arcane Negation

Whenever you would be hit by an Area of Effect spell that deals damage other then psychic, you can reduce the damage you and creatures within 5ft of you would take by 2d4 + Con. After doing so, the next strike you make with a Heavy, Two-handed or Versatile weapon deals an additional amount of damage equal to the negated damage in its original damage type. The die used to decrease the damage increases to a d6 at level 10 and a d8 at level 16.

You cannot use this Reaction if you are not carrying a Heavy, Two-Handed or Versatile weapon. Summoned or otherwise magical weapons can be used, but loose the Con bonus to the negation roll.

You can use this reaction a number of times = to half you fighter level per Long Rest. 

Level 10: 10 Level: Unsettling Presence

Deep, reverberating vocals and strings can be heard within a 10ft Diameter of you. Creatures within struggle to maintain concentration on spells and gain disadvantage on concentration checks. As a Bonus Action, you trigger this aura to appear for 1 minute. As a Bonus Action on subsequent turns, you can force a spell caster within this aura to make a Concentration check.

Once triggered, this aura cannot be triggered again until you complete a long rest.

Level 15: Punish Them

Your attack rolls with weapons and Unarmed Strikes can now score a Critical Hit on a roll of 19–20 on the d20, only when against a creature that has cast a spell this round or is concentrating on one currently.

If such a Critical Hit occurs, you can move up to half your movement to attack a separate creature.

Level 18: Overwhelming Dominance

Cantrip spells cannot effect you. Your movement and voice cannot be reduced or increased by spells of 1st level either.