Magekiller

Whether they're called witch hunters, cult breakers, or magekillers, some rogues specialize in fighting spellcasters.  They run the spectrum from vigilantes with a taste for liquidating necromancers to church-trained assassins who eradicate heretics and rival faiths' priests.  All are subtle and skilled slayers.

Bonus Proficiencies

When you choose this archetype at 3rd level, you gain proficiency in Arcana, Religion, and with the disguise kit.  If you are already proficient, your proficiency bonus is doubled for any checks made using those proficiencies.

Sense Spellcasters

Beginning at 3rd level, you can sense the location of nearby spellcasters.  As an action, you can open your senses to detect magic-users.  Until the end of your next turn, you know the location of any creature within 60 feet of you that can cast spells and is not behind total cover.  You don't learn the identity of such creatures, but you do learn the type of spellcaster they are.  For example, detecting a College of Valor bard makes you hear martial music, while Pact of the Fiend warlocks stink of brimstone.  However, this feature does not detect creatures that have only an innate spellcasting ability.

You can also use this feature to cast Detect Magic without using verbal or somatic components.  You can cast it as a ritual spell without expending a use.  You can use this feature a number of times equal to 1 + your Intelligence modifier. When you finish a long rest, you regain all expended uses.

Magic Resistance

At 3rd level, when you attempt to disarm a magical trap, or make a saving throw against a spell or a monster's magical or supernatural ability, roll a d4 and add the number rolled to the result.

Examples of a monster's magical or supernatural ability include a medusa's gaze, dragon's breath, and mind flayer's mind blast.

Disrupt Dweomer

Starting at 9th level, when you damage a creature that is concentrating on a spell, it has a penalty on the saving throw it makes to maintain concentration.  The penalty is equal to half your proficiency bonus.

In addition, you can cast Dispel Magic using Intelligence as your spellcasting ability and without using any verbal or somatic components.  Once you use this feature, you can't use it again until you finish a short or long rest.

Null Veil

At 13th level, you and the objects on your person cannot be detected by divination spells.  You can choose to exempt certain people from this feature.  For example, you could allow your allies to cast Scrying on you.

Furthermore, as a bonus action, you can turn invisible for 10 minutes or until you attack.  While invisible, you cannot be seen or revealed by spells and monster abilities such as Faerie Fire, See Invisibility, or Truesight.  Once you use this feature, you can't use it again until you finish a short or long rest.

Improved Magic Resistance

At 17th level, you become even more resistant to magic.  Whenever a creature makes a spell attack against you, it must roll a d4 and subtract the number rolled from the result. 

Nullifying Strike

Starting at 17th level, when you hit a creature with a sneak attack, you can force it to make a Constitution saving throw.  The DC is 8 + your Intelligence modifier + your proficiency bonus.  If it fails, until the end of your next turn, it cannot cast any spells or use any magical or supernatural abilities, and any spells or magical or supernatural abilities of your choice currently affecting it or that it is concentrating on immediately end.  This feature affects creatures with an innate spellcasting ability.  Regardless of whether the target succeeds or fails its saving throw, an individual creature can be affected only once every 24 hours by this feature.