Witch hunters are skilled killers who oppose the "dangerous" community of spellcasters around them. They specialise in shutting down the powers of magic users in close combat, but use minor magical powers themselves "to fight the greater evil".

The Watchful Eye

Starting at 3rd level, you learn to discern the presence of a threat to your people. You learn the spell Detect Magic and can cast it a number of times equal to your proficiency bonus. If for some reason required, your spellcasting ability for this spell is Intelligence. You regain all expended uses upon finishing a Long Rest.

Mage Hunter

When you choose this archetype at 3rd level, you learn how to exploit the distraction of spell components, making you a deadly adversary to any mage. Your Sneak Attack damage can be applied to a target even if you don't have advantage on the attack roll, provided that it has cast a spell since the start of your last turn. This damage can not be applied if you have disadvantage on the attack roll.

Spellbreaker

Starting at 9th level, you become able to exploit every drop in a mage's guard. When you deal sneak attack damage to a creature that is concentrating on a spell or similar effect, you may reroll up to half your sneak attack dice. You must take the rerolled results.

Spell Deflection

By 13th level, you have learned enough about the nature of magic that you can change its course as it travels. When you take damage from a spell, you may use your reaction to deflect the brunt of it. You gain resistance to the spell's damage, and you may choose one target within 20 feet of you. The target takes damage of the same type equal to a roll of your Sneak Attack dice. Once you have used this feature, you cannot use it again until you finish a short or long rest. 

Witch’s Bane

When you reach 17th level, your ability to combat mages has become so effective that you can break the casting of a spell as it’s being cast. When you hit a target with an attack that would deal Sneak Attack damage thanks to to Mage Hunter, you can forgo the Sneak Attack damage to instead apply this effect to the target:
• The first time this creature attempts to cast a spell on its next turn, it must make a Dexterity saving throw (DC 8 + your your Dexterity modifier + your Proficiency Bonus), failing to cast the spell on a failed save. The target does not lose a spell slot and may choose another action, but cannot cast spells except cantrips for the rest of the turn.
You may use this feature a number of times equal to your proficiency bonus and regain all uses when you finish a Long Rest.