Witch Hunters, and similarly trained and idealized warriors, have been a footnote in almost every culture's history. Whether acting as champions of the just against ruthless spellcasters or as vile extremists committing genocide against all gifted with magic, these specially trained fighters are relentless in the pursuit of their quarry. They have physically trained to resist and overcome magic of nearly every variety and many have gone even further, specifically learning the best ways to target specific types of spellcasters. 

Seeker

When you choose this archetype at 3rd level, you become proficient in the Arcana skill, or if you are already proficient with that skill, you gain double proficiency. You always know if an object or person you are touching is affected by magic.

Magic Mitigation

At 7th level, pick one of the following elements: Acid, Cold, Fire, Force, Lightning, Necrotic, Poison, Psychic, Radiant, Thunder. You are now resistant to that element. At 15th level you are immune to that element.

Additionally, you have advantage on any Wisdom saving throws against magic effects. 

Specialized Mageslaying

At the 10th level, you can choose a particular type of spellcaster (Bard, Cleric, Druid, Eldritch Knight, Paladin, Ranger, Warlock, Wizard) to be especially effective against. Pick two cantrips, one level 1 spell, and one level 2 spell from that class's spell list. The chosen spells must be single-target and have an offensive effect (i.e. Dissonant Whispers, Fire Bolt, Guiding Bolt, Heat Metal, etc.) When you succeed on a saving throw against one of the selected spells you take no damage and suffer no negative effects. Enemies have disadvantage on their ranged spell attacks against you if it is one of the spells you chose.

If a creature casts one of the chosen spells and it successfully damages you, you have advantage on your next attack against the creature.

Zealous Pursuit

Beginning at 15th level, you ignore all negative effects of difficult terrain caused by magic, as well as spell effects that might cause your movement speed to be lowered. Additionally, if a creature within melee range of you casts a spell with an invisibility, teleportation, or healing effect, you can use your reaction to make a punishing melee attack that interrupts the ability, negating its effect if it hits and causing additional psychic damage equal to half your fighter level (rounded down) to the spellcaster.

Mage Assassin

At the 18th level you achieve the pinnacle of Witch Hunting. Based on your fighting style, you gain one of the following effects:

Absorbing Barrier (Protection)

When a creature you can see casts a spell against a target other than you within 30' of you, you can use your reaction to goad the enemy into attacking you. Until the end of its next turn, the enemy has disadvantage on all single-target spell attacks against targets other than you.

Additionally, if you are wielding a shield, whenever a single-target ranged spell attack against you "misses," it is absorbed by your shield. Upon reaching three absorbed spells you can choose to throw your shield as an action. When you throw the shield, select a target you can see within 30 feet and up to two additional targets within 10' of each sequential one. Make a separate ranged attack for each shield strike. Each successfully hit target takes 3d4 force plus 1d4 magical bludgeoning damage. After the shield hits or misses the last target it rebounds back to you, possibly hitting all previously struck targets again. 

Allez (Dueling)

Gain +5 to your initiative rolls. Additionally, you always succeed on saving throws against spells cast by a creature within melee range. If a creature casts a single-target, melee spell against you, make a Dexterity saving throw versus their spellcasting save. On a successful save, you deal double your weapon's full damage to the spellcaster.

Dervish (Two-Weapon Fighting)

On your turn you can choose to use your bonus action to begin spinning wildly until the start of your next turn. While spinning, you are immune to all single-target weapon and spell attacks. While spinning you cannot take any reactions. Any single-target spells or projectiles that try to hit you are reflected back at the source, dealing one-quarter damage (rounded up). Melee weapon and spell attacks are harmlessly deflected. If a creature moves to or is within melee range of you, they must make a Dexterity saving throw versus your Dexterity DC (8 + proficiency + Dex) and take both sets of rolled weapon damage on failed saves, or half as much damage on successes. This spinning effect ends if you are incapacitated.

When the effect ends, your movement speed is reduced by half until the end of your turn and your next attack action has disadvantage.

Reflective Armor (Defense)

If you are affected by a single-target spell that deals damage to you, the enemy takes half that damage as well. If the spell has an effect that lingers beyond the initial damage, the enemy is also affected equally.

Wicked Strength (Great Weapon Fighting)

Your two-handed melee weapons are considered magical for the purpose of overcoming resistance to non-magical weapon damage. Additionally, when you successfully attack a creature with a two-handed melee weapon, they cannot take reactions against you, including casting the Shield spell. If the targeted creature has cast any spells since your last turn, they take an additional weapon die of damage from your two-handed weapon melee attacks.

Witch Sniping (Archery)

When you select this option, your ranged weapon becomes blessed with antimagical power. When you successfully hit an enemy beyond melee range, they take an additional 1d6 psychic damage and they are knocked back 1d4 * 5 feet. If you have advantage on the ranged attack, the target must make a Strength saving throw or be knocked prone. Enemies successfully hit by your ranged weapon attacks have disadvantage on any concentration checks to maintain spells until the end of their next turn.