Mage Hunter

An ancient order that remembers the ruin that can be wrought by spell-casters keeps an eternal vigil. Watching those gifted with magic talent for any sign of becoming a danger to civilization. These Mage Hunters are trained from a young age to serve, their minds and bodies tempered into keen weapons. Mage hunters undergo rigorous training and have their flesh marked with runes to ward off magic. They are the vigil, ever watchful, poised to strike any unsuspecting mage who's hubris grows too far.

Hunter Training

When you choose this archetype at 3rd level, you gain proficiency with the Arcana skill. Additionally you gain proficiency in the heavy crossbow, scimitar, and whip

Arcane Assassin

Starting at 3rd level, you use your knowledge of arcana to exploit their weaknesses. You have advantage on weapon attacks made against known spell-casters.

You must have seen the spell-caster cast a spell, use an item, or somehow otherwise act in a manner that identified them as a spell-caster to gain this benefit.

Null Magic

Starting at 9th level, you gain the ability to use your knowledge of the arcane to unweave the magic of your enemies. You can cast the counterspell or [Tooltip Not Found] spells. Your spellcasting ability for these spells is Intelligence. The spell slot level used to cast these spells is considered to be 1/3rd your Rogue level (rounded down).

This ability can be used a number of times equal to 1 + your intelligence modifier. You regain all uses of this ability upon completion of a long rest. 

Steady Eye

Starting at 13th level, you gain the ability to innately detect magic. You can recognize magical relics, items, and spell-casting paraphernalia such as spell components, arcane focus, or spell-books on sight. you don't know their purpose or enchantments, only that they are magical or sere some purpose in the work of spell-casters.

Additionally you can use Intelligence investigation to attempt to decipher magical script or the purpose of magical objects.

Spellpiercer

 

Starting at 17th level, when you are attacking a creature that is under the effect of a spell that grants a bonus to AC or resistance to damage, you ignore those benefits while making attack or damage rolls. This does not affect spells that reduce your  ability to hit or damage such as Bane, Darkness, or Invisibility. 

When dealing damage you also ignore temporary hit points granted by spells, dealing damage directly to the creature's normal hit points instead. The temporary hit points are unaffected.

Examples of spells that would be ignored by this ability are: Aid, Armor of Agathys, Barkskin, Blade Ward, Blur, False Life, Heroism, Mage Armor, Shield of Faith, Stoneskin, and Warding Bond.