Spellslayer

At a party, a gnome dispels the disguise worn by a dark sorcerer, the guard already on alert, breaking the mages plans and leading to their execution. 

Deep in the woods, a tabaxi stalks her prey. The hag has taken a half dozen, but the fey doesn't know that its not the only hunter out here.

A half-elf steals an artifact from an evil priest, and plants evidence pointing blame. Without it, this bloody cult will collapse from infighting and suspicion, and the town will be safe from their necromantic magics.

Nulknights, Mage Hunters, Inquisitors, Wardens of the Silent Dawn, Spellslayers. 

Whatever you call them, Spellslayers are not to be taken lightly. Either through an Order, a church, or a personal vendetta, Spellslayers take up arms and to the shadows to seek out and hunt down magic at its sources. They may be protectors of the Material Plane, they may have taken an oath to not allow common folks to come to harm, or they may have once lost loved ones to a dark ritual or spell gone wrong. They learn of magic so they may seek out and counter it, leaving behind the wordless warning that no mage is above censure, and no harm coming to the land and common people is tolerated. 

Knowledgable Hunter

When you choose this archetype at 3rd level, your studies of mages, the planes, tracking, and mortal behavior have begun to pay off. You become proficient in Arcana, and Investigation.

Arcane Sense

Starting at 3rd level, you become more in tune with traces of spellcasting. Using a bonus action, the Spellslayer can identify if any target it can see is concentrating on a spell. Sneak attack is granted against the concentrating target, regardless of if the target was previously eligible for sneak attack.

Nullifier

At 9th level, your fight against magic and studies of arcane prevention have greatly progressed. Once per long rest you can cast Counterspell as if it were a level 5 spell. 

You gain an additional use of this at level 15.

Iron Will

You are harder to baffle, compel, or manipulate by magical means. Starting at 13th level, you have advantage on any saving throw related to magical effects governed by Intelligence, regardless of if you are proficient in that saving throw. This feature is only effective while you are conscious and not suffering from exhaustion or poison. 

At level 19, this extends to when you are asleep or incapacitated. 

Spell Breaker

Beginning at 17th level, magic spells stand no chance against you. You now have access to the spell Dispel Magic. Additionally, you can now use Detect Magic as a ritual spell.

For the purposes of use, the spells are treated at their level base. You can cast Dispel Magic without verbal or somatic components from a spell pool equal to 1 + your Intelligence Modifier. You regain all expended uses of it when you finish a long rest.

Detect Magic can only be used as a ritual spell twice a day and does not count towards using a slot from the spell pool. 

Master Spell Breaker

You've mastered breaking spells and can add double your intelligence modifier to dispelling higher levels spells. This feature can only be used once per long rest.