Oath of the Coven.

Nobody is more aware of the incredible power of witchcraft than witches themselves. Witches form covens comprised of powerful spellcasters and formidable warriors, covenants that grant them tremendous power, and bonds that share their power between one another. It is no wonder that with magical groups wielding such might, someone is needed to keep the peace.
Paladins who swear to follow the Oath of the Coven are there to protect witches—most of the time, to protect them from other witches. Despite the wide diversity of covens’ values and teachings, nearly all of them include at least one Oath of the Coven Paladin. They are there to keep the peace and to protect other witches from outside threats, but also to ensure that their own companions do not go mad with power. Many are sent after witches who have broken the law, killed their masters, or otherwise endangered the well-being of other witches. Once an Oath of the Coven Paladin is on the hunt, very little can stop them.
These paladin witches use their witchcraft to stop spellcasters in their tracks. They are uniquely suited to counter and take down those who use magic to harm or manipulate others, and their abilities centre around interrupting or resisting spells. Many witches are thankful for their presence, while many others resent them and see them as an unnecessary precaution stifling their pursuits in spellcasting, but witches have learned that a steady hand holding an unwavering sword can be the only thing stopping a coven from crumbling under its own weight.

Witchmark.

Witchmarks for Oath of the Coven paladins tend to take the form of family crests, since many of these paladins come from ancestral family lines. Other paladins have witchmarks in the shape of scars caused by damage-dealing spells, while others have the text of their own oaths as witchmarks snaking around their arms.

Level 1 Features:

Tenants of the Coven.

The tenets of the coven vary by which coven a paladin is entrusted to protect, but all tenets revolve around punishing those who use magic for evil. Those who swear this oath are willing to do whatever it takes to stop those using magic to make others suffer.
Punish the Wicked, Help the Good: I am the sword that punishes those who use magic for evil. I am the shield that protects those who use magic for good.
Seek out the Wicked: Nothing in this world will deter me from seeking and finding those who have committed magical atrocities.
Neutralize or Exterminate: I will not let those who have committed magical atrocities go free without paying for their crimes.
Magic is Neutral: Magic, like nature, is a neutral force that can be used for great good or great evil. I will never make the mistake of seeing magic or those who practice it as inherently wicked.

Level 3 Feature:

Oath Spells.

Each oath has a list of associated spells. You gain access to these spells at the levels specified in the oath description. Once you gain access to an oath spell, you always have it prepared. Oath spells don't count against the number of spells you can prepare each day.
If you gain an oath spell that doesn't appear on the paladin spell list, the spell is nonetheless a paladin spell for you.

Channel Divinity: Divine Dispel.

As an action, you force any creatures of your choice you can see that are using magic to put a creature under a magical effect, including but not exclusively charmed, frightened, or restrained, to make a Charisma saving throw against your spell save DC. On a failure, the magical effect ends.

Channel Divinity: Spell Lock.

When a creature casts a spell, you can use your reaction to prevent that creature from casting that same spell again. As a reaction, when a creature that you can see within 60 feet of you casts a spell, you can force it to make a Charisma saving throw against your spell save DC. On a failure, that creature is unable to cast the spell that triggered this reaction. They can repeat their saving throw at the end of their turns.

Level 7 Feature:

Aura of Antimagic.

Beginning at 7th level, your very presence disrupts the flow of magic in your vicinity. Hostile creatures within 10 feet of you have disadvantage on their concentration saving throws, and you and friendly creatures within 10 feet of you have advantage on Charisma saving throws.
At 18th level, the range of this aura increases to 30 feet.

Level 15 Feature:

Ward of the Antimage.

Starting at 15th level, you become even more resistant to any spell cast upon you. If a spell forces you to make a Wisdom or Intelligence saving throw, you can roll a Charisma saving throw instead of the saving throw the spell calls for.
Additionally, you can see through magical darkness and can’t be slowed by magical difficult terrain

Level 20 Feature:

Witch Knight.

At 20th level, you can summon the spirits of those who have suffered at the hands of evil mages to embolden you in your crusade against those who abuse their powers. Using your action, you undergo a transformation. For 1 minute, you gain the following benefits:
-You have resistance against damage from all spells.
-All spells you cast are treated as though you cast them at one spell slot level higher than the spell slot you actually used to cast the spell. For example, a level 1 spell cast with a level 1 spell slot behaves as if it was cast with a level 2 spell slot.
-You and friendly creatures within 30 feet of you can’t be blinded, charmed, deafened, frightened, paralyzed, or petrified by magical means.
Once you use this feature, you can’t use it again until you finish a long rest.