These marks assume they are rarer but more powerful features and subclasses are not based on the target
You specialize in hunting those who utilize spellcasting.
Spell Resistance. You have advantage on saving throws against spells.
Arcane Conduit. Once per turn, when you hit a target with an attack, and that target is under the effects of a spell that a creature is concentrating on (it can be the same creature or a different one), you can choose to deal 1d6 plus your Wisdom modifier force damage to the creature concentrating on the spell, as you send a pulse of destructive energy along the magical connection.
Arcane Disruption. As a bonus action you throw a silvery dust into the air in a 10-foot cube originating from you. Until the start of your next turn, spell attacks have disadvantage against creatures within the area.
Animal Friend. You have proficiency in animal handling.
Summon Steed. You can cast the find steed spell at will.