The anti magic rogue is someone who developed very skillfull ways of preventing casters to use their magic. By slicing very specific spots of the body, or stealing/destroying the focus and predict how the ennemy caster is gonna act, he's their natural ennemy. 
Often part of an organisation despising magic users, mostly for their arrogance and bulying of the weak (and non magic users), the anti magic rogues analyzed the functionning of magic, and the body of magic users to determine how to counter them. They discovered weak points on the magic users body, which when damaged makes it far harder for them to practice magic. They also have strategies to disturb them, and have a lot of experience dealing with them and their magic attacks.
They work agains any and all kind of magic conglomerates, and try to act as a kind of "protector of the weak".

Focus the focus

Starting at 3rd level, if in melee range, you can use an action to steal a casters focus with a dexterity (sleight of hand) check, even in combat, or destroy it with an attack roll (works with ranged attack).

Beware of magic

When chosing this archetype at 3rd level, you have advantage on saving throws against spells, when you can see the caster launching the spell.

Disabling Cut

Starting at 9th level, when you land an attack on a spellcaster, you can choose to do minimum dice damage. If you do so, the spellcaster will have to do a constitution saving throw (DC 8 + Dexterity modifier + profiency bonus) at the end of each of his turns. Until he succeds, he is unable to cast any spells.

Anti Magic Expertise

When you reach 13th level, you have mastered your art. You can now use focus the focus as a bonus action, and apply Disabling Cut on crits, therefore ignoring the minimum damage condition.

Knife Lord

Starting at 17th level, you have become a master of the knife, and can precisely hit any part of someones body. You can apply the disabling cut to anyone, and choose between its initial effect, or apply the blinded status, deafened status or reducing the speed to 0 (only one of these). Whatever you chose, they still have to succed the constitution saving throw to recover.