Pact of the Knight

You have made a contract with a high ranking member of an Extraplanar Court. Gaining many of the same features enjoyed by so many other Warlocks, such as regaining your meager spell slots on a short rest. Access to once per day powerful spells through your pact once you are able to cast above fifth level. But the core features of your particular pact are quite unique. The Eldritch Blast employed by other Warlocks is just that a blast of concentrated magic. In your hands however, the contract shapes this wonderfully powerful cantrip into a Blade. Attacks made with what is now known as Your Eldritch Blade are simple weapon spell attacks (Simple Weapon, that uses your Spell Attack Modifier for Attack Rolls) that you are proficient with. By making this pact you will no longer be able to cast Eldritch Blast as normal. Instead at levels 5, 11, 17, when it would gain an extra beam you are granted an Extra Attack when you take the Attack Action with your Eldritch Blade. These Extra Attacks are only gained through attaining the specified levels in the Warlock Class. Other instances of Extra Attack do not confer normal Extra Attack features to the Eldritch Blade which is the symbol of your pact. For example gaining 5 levels in Warlock will grant 2 attacks per Attack Action with the Eldritch Blade. But, Reaching level 5 in Warlock and Level 11 in Fighter you will still only have 2 Attacks per Attack action with Eldritch Blade, Any other weapon will still gain the benefits of the Fighter Levels.

The Eldritch Blade is a Two-Handed Weapon with variable properties based on the Invocations taken at the appropriate Warlock levels. 

Agonizing Blast: Does not affect Eldritch Blade

Repelling Blast: Forces a Strength Save vs Your Spell Save DC or be Knocked Prone in addition to the normal damage.

Grasp of Hadar: Forces a Charisma Save vs Your Spell Save DC or be subjected to a Paralysis effect until the end of your next turn.

Eldritch Spear: Grants Your Eldritch Blade the Reach Property.

Lance of Lethargy: As a Reaction, when a creature attempts to leave your threatened area you may make An Opportunity Attack that if it hits will reduce the target's movement to 0 until the start of their next turn.

The Eldritch Blade is also able to be chosen as your Pact Boon of Pact of the Blade. However, the Eldritch Blade is not compatible with the following Invocations.

Improved Pact Weapon 

and

Thirsting Blade

This pact also confers upon you a Natural and improving over time Mage Armor that includes both your Dexterity and your Charisma modifiers in the calculation to determine your Armor Class. Whenever your Eldritch Blade gains an Extra Attack your Mage Armor improves by +1

Knightly Vestments Growth Calculations:

Warlock Level:
Level 1: 13+Dexterity+Charisma
Level 5: 14+Dexterity+Charisma
Level 11: 15+Dexterity+Charisma
Level 17: 16+Dexterity+Charisma

Expanded Spell List

Spell Level:
Level 1: Shield, Compelled Duel, Command, Heroism, Wrathful Smite
Level 2: Branding Smite, Spiritual Weapon
Level 3: Crusader's Mantle

Starting at Warlock Level 6 you will be able to spend one of your Spell Slots as a Bonus Action to transform your Eldritch Blade from Force Damage to any one of the other Damage types you currently have access to via your Known Spells.

At Warlock Level 10 You will gain Resistance to Force Damage and You may add your Charisma Modifier to All Dexterity Saving Throws.

And at Warlock level 14 Your ability to change the damage type of your Eldritch Blade shifts once more and When you spend a Spell Slot to change the damage type, your Eldritch Blade's damage dice increases to 2d10. 1d10 is Force Damage and the other 1d10 is the chosen Damage type.

Knightly Vestments

"The Clothes make the man." They say. And in this case, You gain, when Unarmored a Natural Mage Armor that grows with you. Whenever your Eldritch Blade improves, at levels 5, 11, and 17 your Armor Calculation changes in the following ways:

Level 1
13+Dexterity+Charisma

Level 5
14+Dexterity+Charisma

Level 11
15+Dexterity+Charisma

Level 17
16+Dexterity+Charisma

It is considered a Persistent Effect for the terms of Dispel Magic.

Bonus Cantrip - Eldritch Blast

While it's true the Eldritch Blast Cantrip is already on your Spell List, This Subclass adds it automatically to your Known Cantrips List. However, You, The Pact of the Knight Warlock, do not cast this Cantrip in the Usual Manner.

See the Eldritch Blade feature for more details on how you are now casting Eldritch Blade.

Eldritch Blade

At the start of Combat, as a Free Action, you may channel the Arcane Power granted to you by your pact to create your Eldritch Blade. By default it resembles a standard Longsword made of pure energy. Its appearance does not affect its damage. The blade summoned in this way forms around your Arcane Focus which forms the hilt. It functions as a Two-Handed Longsword without the Versatility Property. 

When you gain levels in this class Your Eldritch Blade will improve over time. At level 5 you will gain the Extra Attack Feature exclusively for the Eldritch Blade. And again at levels 11 and 17, These Extra Attacks only apply with the Eldritch Blade and only when you acquire the corresponding levels in this class.

Blade Change

As a Bonus Action When summoning your Eldritch Blade you may spend a Spell Slot to change the Damage Type from Force to any other Damage type you have access to through your Known Spells.

Knight's Bulwark

Through constant training and use of your Eldritch Blade, you have become more accustomed to the way Force Damage works. This training and experience has granted you Resistance to Force Damage. This training has also shown you how to more effectively dodge incoming Spell Effects.

Improved Eldritch Blade and Blade Change

When taking the Bonus Action to spend the Spell Slot to use the Blade Change feature The Damage of Eldritch Blade increases to 2d10. 1d10 is Force Damage and 1d10 is the chosen Damage type.