Way of the Mage Slayer

A human wizard attempts to cast a powerful spell, but a trained tiefling quickly interrupts the flow of magic in his body.

A dragonborn sorcerer hits a halfling monk with a lightning spell, which the halfling shrugs off with ease.

An elven warlock is shocked when his tabaxi opponent turns his own spell against him.

Monks trained in the Way of the Mage Slayer are trained to hunt down and immobilize those of arcane nature that align towards evil, and even trained to kill those who cannot be redeemed. 

Arcane Defiance

At 3rd level, your ki flow provides protection against specific magical sources. When taking a long rest, you can meditate to gain resistance from one of the following magic damage types; Cold, Fire, or Poison. 
When you gain more levels in this subclass, your damage types expand.

6th level - Lightning, Thunder, Acid
11th level - Psychic, Radiant, Necrotic

Additionally, when meditating for resistance, you also gain a damage type when attacking from the list, whether it's the same or a different type. You deal the elemental damage type only when you manage to strike an enemy with a ki based attack. The damage of your magical damage equals one of your martial arts die.

 

Advantageous Mystic

Starting at 6th level, you are an adept at countering spellcasters. Whenever a creature within 5 feet of you casts a spell, you can use your reaction to attempt to stop the spell. You must spend 3 ki points. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect If it is casting a spell of 4th level or higher, make a Wisdom ability check. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.

Additionally, any creature you hit with a ki based attack has disadvantage on Constitution saving throws for 1 minute.

Magic Opportunist

At 11th level, you can harness some of the magical prowess for protection and have increased offense. As a reaction against a melee attack or when targeted by the magic missile spell, you can spend 1 ki point to raise your AC by 5 until the start of your next turn, taking no damage from magic missile.

Additionally, if you make a melee weapon attack against a creature that can cast spells, you can use your bonus action to move them up to 10 feet in any direction you choose, using 1 ki point.

Sorcerous Scourge

By 17th level, you are a dominating force against spellcasters. Whenever you hit a creature with a ki based attack, you delete their ability to cast spells. The creature must make a Constitution saving throw against your ki save DC. On a failed save, they lose spell slots up to a number of ki points spent on the attack (up to your wisdom modifier) or half as much on a successful save (minimum of one spell slot). They begin losing spell slots starting from 1st level.

Additionally, your magical damage type from Arcane Defiance ignores any resistances.