The Nullifier 5.5

An Anti-Magic Warrior. The Nullifier subclass bases itself around a dedicated dislike for enemies of a magical variety. Dealing extra damage, making it easier to save against mind altering spells and even stopping mages directly in their tracks by stopping their casting of schools of magic.

Level 3: Anti Magic Strike

You have trained to exploit the mental weaknesses of creatures that wield magic. Once on each of your turns, when you hit a creature with a weapon attack as part of the Attack action, you can deal extra Force damage to the target. The extra damage equals 1d4 plus the target's modifier for one ability score of your choice: Intelligence, Wisdom, or Charisma.

At 10th level, the extra damage increases to 1d6 plus the target's chosen ability modifier.

Level 3: Arcane Sleuth

Your pursuit of spellcasters has taught you to recognize the signs and workings of magic.

You gain proficiency in the Arcana skill. If you already have proficiency in Arcana, you instead gain Expertise in that skill, allowing you to add twice your Proficiency Bonus to ability checks you make with it.

In addition, you have Advantage on Intelligence (Arcana) checks made to identify, examine, or understand a spell that is currently affecting a creature or object, or that you can perceive a creature casting. To gain this benefit, you must be able to see or hear the creature or magical effect, and it must be within 60 feet of you.

Level 7: Magical Endurance

Your relentless training has hardened you against magic directed at you alone.

Whenever you make a saving throw to resist a spell or magical ability that targets only you, you gain a bonus to the saving throw equal to your Constitution modifier (minimum bonus of +1).

Stifle The Black Arts

You have learned to disrupt spellcasting through precise, forceful intervention.

When a creature you can see within 5 feet of you begins casting a spell, you can take a Reaction to attempt to interrupt it. Make a Strength (Athletics) or Dexterity (Acrobatics) check. The DC equals 10 + the spell's level.

On a success, the creature's spell fails, and the spell has no effect. Any spell slot or other resource expended to cast the spell is still expended.

You can use this feature a number of times equal to half your Fighter level, rounded up. You regain all expended uses when you finish a Short or Long Rest.

Level 10: Fortified Mind

Years of resisting magical influence have fortified your mind against supernatural intrusion. You have Advantage on Intelligence, Wisdom, and Charisma saving throws.

Level 15: Mystical Tenacity

Your determination allows you to pursue spellcasters even as they attempt to keep their distance.

When you use your Stifle the Black Arts feature, you can move up to half your Speed toward the creature attempting to cast the spell as part of the same Reaction. This movement occurs before you make the ability check and doesn't provoke Opportunity Attacks from that creature.

Force Mundanity

Your strikes disrupt the connection between a creature and the arcane forces it commands.

Once per turn, when you hit a creature with a weapon attack, you can choose two different schools of magic. The target must make a saving throw using its spellcasting ability. The DC equals 8 + your Proficiency Bonus + your Strength or Dexterity modifier (your choice).

On a failed save, the target can't cast spells from either of the chosen schools until the end of its next turn.

This effect ends early if you use this feature on another creature.

Level 18: Sunder The Weave

Your ability to disrupt magic has become so powerful that a failed spell can rebound upon its caster.

Whenever you succeed on the ability check for your Stifle the Black Arts feature, the creature whose spell you interrupted takes Force damage equal to 1d12 per level of the spell it was attempting to cast.

If the interrupted spell is a cantrip, the creature instead takes 1d12 Force damage.