Crusader

Those who bear the mantle of the Crusader were trained in their art for a specific purpose, or selected on merit to pursue that purpose.  That purpose is kill-- to kill if need be, or to kill as an end in and of itself.  While all fighters are trained with arms, a Crusader's study is focused on combat against a specific foe, be it dragons, the Unholy, or man.  A Crusader is or once was bound to a singular mission, and is driven to push on through the pain of mortal injury.  Some Crusaders follow codes of honor and chivalry, while others are merciless machines, consumed by blood and war.

Martial Study

When you choose this archetype at 3rd level, you have trained and studied extensively to defeat a specific foe.  Choose one the following creature type options:

• Humanoids
• Dragons
• Giants
• Celestials
• Unholy (fiends and undead)
• Unnatural (aberrations and monstrosities)
• Wild (beasts, plants, fey)
• Arcane (constructs, elementals, oozes)

You have advantage on Intelligence ability checks to recall information about creatures of the chosen type.  When you deal damage with a melee weapon attack to a creature of the the chosen type, you deal an additional 1d4 damage; if your chosen creature type is dragons or celestials this benefit extends to ranged weapon attacks.  You have advantage on ability checks made to grapple or shove creatures of the chosen type, or to escape grapples by such creatures.

Grit

Starting at 3rd level, your instincts drive you to push on through mortal injury.  Being reduced to 0 hit points doesn’t knock you unconscious.  You still must make death saving throws, and you suffer the normal effects of taking damage while at 0 hit points.  Additionally, when you take damage while at 0 hit points, you must succeed on a Constitution saving throw with a DC of 10 + the damage taken or fall unconscious.

Starting at 7th level, the DC is equal to the damage taken.

Zealous Leader

Starting at 7th level, you have advantage on Charisma saving throws, saving throws against being charmed or frightened, and saving throws made to avoid taking levels of exhaustion.  Additionally, friendly creatures within 30 feet of you that can see or hear you have advantage on saving throws against being frightened while you are conscious.

Zealous Smite

Starting at 10th level, once per turn when you deal damage to a creature using your Martial Study feature, you can deal an additional 1d12 damage.  You can use this feature a number of times equal to your proficiency bonus, and regain all expended uses when you complete a long rest.

Starting at 15th level, you gain temporary hit points equal to your proficiency bonus when you deal damage using this feature, or temporary hit points equal to twice your proficiency bonus if you have 10 or fewer hit points remaining. 

Restless Heart

Starting at 15th level, your body resists forces that would hold you from your goal.  You are immune to being frightened.  You have advantage on saving throws against being paralyzed or unconscious, and magic cannot put you to sleep.

Armored Juggernaut

At 18th level, you are a master of using your armor to deflect swords, halberds and claws.  While you are wearing heavy armor, bludgeoning or slashing damage that you take from a source other than fall damage is reduced by 2, and you have resistance to piercing and slashing damage.