Mage Eater

As a rogue, your skills are particularly niche. As a Mage Eater, you are a bane to the existence of magic everywhere. 

Spell Eater

When you reach 3rd level, you gain the ability to rend magic from the caster. Whenever a creature within 30 feet of you casts a spell, you may use your reaction to make an attack against that creature. The target must then make a Constitution saving throw with a DC of 10 + the spell level or lose the spell. This attack does not get sneak attack damage. 

If the spell fails, you receive a bonus to your next attack roll equal to the spells level.

Forced Stop

3rd-level Mage Eater feature

Casters are deceptively defensive. Immediately after your attack is magically blocked, such as by the shield spell, or impeded such as with wall of force, roll half the number of Sneak Attack dice for your level (round up), and the creature takes force damage equal to the roll’s total.

You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

Master Identifier

Starting at 9th level, you may make an investigation check to search for residual magic. As a bonus action, you may investigate an area, and discover any magical traps, effects, and barriers that are not invisible. If they are invisible, you know that something is cloaked but not what. You do know the school of magic, the effect, and the level of any spells produced by the effect. 

If the effect or spell is item based, you do not know the item or any curses attached to it, but you can identify that it is an item.

Versatile Trickster

At 13th level, you gain the ability to prepare not only physically but mentally for attacks. Whenever you take the Dodge action, you gain advantage on any saving throws against spells.

Caster Counterstrike

At 17th level, you are able to counterstrike against a caster.

Immediately after a creature casts a spell that targets you or includes you in its area of effect, you can use your reaction to make an attack against against the creature.

You may do this once per long rest.