Path of the Mageslayer

Regardless of how magic comes into existence, the effect of magic is pretty constant.  It changes something in nature into something else, whether this is creating things that aren't there, manifesting energy from nothing, or sending thoughts from one mind to another.  For some reason, these changes do not seem to affect you the way they affect others.  It might be that your connection with the primal forces of this world are so strong, the way things are meant to be in nature overrides any changes forced by magic.  It might be that your rage so focuses you on your present condition, changes affecting you find no purchase.  Or it might be that you are simply so idiotic that anything you do not understand, simply doesn't exist for you.

Magical Resistance

When you select this path at third level, you gain an expanded awareness of magic affecting you, and protection from it.  Once per short rest, you may cast Detect Magic.  While you are in a rage, you gain resistance to all damage from spells affecting you, in addition to all other benefits granted by your rage.

Cleave Spell

At sixth level, you gain the ability to wield your resistance as a weapon against mages.  Once per short rest, you may cast Counterspell at a level equal to your proficiency bonus.  Use Constitution as your spellcasting ability modifier.

Magehunter

At tenth level, you become an expert at fighting mages.  Your rage now grants you an improved resistance to spells.  While raging, you gain advantage on all saving throws against spell effects, and if a spell's effect allows you to make a saving throw to take half damage, you take no damage instead on a successful save.  These effects are in addition to all other bonuses granted by your rage.
In addition, if a creature casts a spell within your threatened area, you may use your reaction to make a melee attack against them before the spell resolves. You make this attack with disadvantage, but if you hit the creature must make a Constitution Saving Throw (DC 10 + your damage). If the creature fails, their spell has no effect and the spell slot is expended.

Magekiller

At fourteenth level, you have become nearly immune to magic.  You are a terrifying figure to any spellcaster, who see in you nothing but impending doom.  When you enter a rage, you may cast Antimagic Field on yourself as a bonus action, lasting until the end of your rage.  Any creature attempting to cast a spell within this zone takes 1d6 force damage per level of spell slot expended (minimum of 1d6).  Once your rage ends, you suffer one level of exhaustion.
You may use your bonus action while in a rage to channel your resistance into a spellcaster's body, giving it mana poisoning.  The spell save DC of the creature is reduced by your Constitution modifier, and the creature has disadvantage on spell attack rolls.  In addition, whenever the creature casts a  spell, it takes 1d6 poison damage per level of spell slot expended.  This effect ends with a successful DC 18 Constitution Saving Throw made at the beginning of the creatures turn, or if it is targeted by a Greater Restoration spell.  You regain this ability after a short rest.