Mage Slayer

Mage slayers wield both a wealth of knowledge about magic and an incredible distaste for its usage. You understand, likely better than most casters you travel with, how magic corrupts and how it can be corrupted. Many mage slayers come from organizations that operate from the shadows, seeking to topple entrenched magical power structures, but it's also a study that can be undertaken as an individual, an alluring path for those who have been burned before.

Mages would place themselves at the top of the world. You exist to bring them tumbling to earth.

Spell Sense

When you choose this archetype at 3rd level, you gain proficiency in Arcana. When you are trying to determine the nature of a spell affecting a creature you can observe, you make Arcana checks with advantage.

Caster's Bane

Starting at 3rd level, you can exploit the focus that spellcasting requires. In addition to normal prerequisites, you can use Sneak Attack on a target who is concentrating on a spell.

Slayer's Retaliation

Starting at 9th level, you can exploit the scant moments when a caster leaves themself defenseless. When a creature casts a spell and is within the normal range of your weapon, you can use your reaction to make an opportunity attack against them. A caster has disadvantage on checks made to maintain concentration against the damage of this opportunity attack.

Slayer's Shield

At 13th level, you gain the ability to deflect and reduce spells cast at you. You gain a number of uses of the following features equal to your proficiency bonus, which can be used so long as you are not incapacitated:

• Agile: You can use your Evasion feature for any saving throw made to reduce the damage of a spell.
• Dispel: As a reaction when a spell targets you, you can attempt to counterspell, using your Dexterity in place of a casting stat. If you succeed on this counterspell, the spell is not canceled entirely, but instead simply fails to affect you.
• Brace: At the start of your turn, you can choose a school of magic. Until the start of your next turn, you gain advantage on saving throws against spells of that school. 

You regain any expended uses of this feature when you finish a long or short rest.

Fate's Parry

Starting at 17th level, you can focus and perfectly reflect a spell. When a creature targets only you with a spell, you can force the spell to target another creature you can see that falls within the spell's range, including the caster. Once you do so, you can't use this feature again until you finish a short or long rest.