Deathless Archetype

Some warriors battle as though they are already acquainted with the grave. As still as a corpse until they strike with sudden power and swiftness, unbending in combat even when injured or nearly dead,  these fighters are known of in legends as Deathless. Some have already died and been returned,  perhaps even as undead. Others are warded against death from birth, or artifice  such as being favored by a god of death. Yet, most Deathless are simply too tough - or too lucky - to die so easily.

Endless Toil

Beginning when you choose this archetype at 3rd level, you have advantage on saving throws against disease, and you can't gain exhaustion from physical exertion, such as by traveling, engaging in combat, grappling or restraining a creature larger than you, or lifting or throwing heavy objects. 

(You can, if your DM allows it, choose the replacement ability for endless toil, which replaces your exhaustion clause with a resistance to one damage type from the following: Acid, Cold, Fire, Poison, Lightning, Necrotic, or Thunder.  When you choose a resistance, you also gain a vulnerability.)

Deathly Vigor

At 3rd level you become a tireless warrior in battle, capable of recovering from injuries without slowing your assault and your bursts of vigor grant a speed to your attacks that would exhaust other warriors. 

When you use a bonus action to regain hit points via your Second Wind feature, you also gain temporary hit points equal to 5 + half your fighter level, and you can make one weapon attack as part of the same bonus action. When you reach 7th level you gain advantage on this attack. 

When you start your turn and have already expended your Second Wind feature, you can choose to regain taking one level of exhaustion when doing so. You can't use this feature again until you finish a long rest. (This long rest cannot also restore your level exhaustion.)

One Foot in the Grave

Starting at 7th level, you can negate necrotic or poison damage (unless you are vulnerable to it) up to half your fighter level, rounded down a number of times equal to your Constitution modifier per short rest. You can reduce one level of exhaustion in a short rest instead of a long one, and your grim and ghastly disposition also gives you advantage on Charisma (Intimidation) checks, but not towards Celestials, Constructs, Fiends or Undead.

Grim Riposte

At 10th level, your deathly stillness can suddenly give way to sudden bursts of frenzied, ghoulish motion that surprise your foes when their guards are down. When you are hit with a melee weapon attack and you have a melee weapon or shield, you can use your Reaction to parry the strike, forcing the attacker to reroll and use the new result. If this ability causes the attack to miss, you can make one melee attack as part of the same reaction against the creature that made the attack. Once you use this feature, you cant use it again until you finish a short or long rest.

Tireless Mastery

Starting at 15th level, you have advantage on weapon attacks granted by your Grim Riposte feature, and whenever you roll initiative and the feature is expended, you regain the use of it. You also gain a bonus to initiative equal to your Constitution modifier, on a natural 20 initiative roll, during the first round of combat you may take another turn with your initiative counted as half your initial roll rounded down.

The Undying

At 18th level, you become something between life and death. You gain the following benefits, each requiring a long rest to use again.

• You no longer need to sleep, and sleeping confers no benefit to you. You cannot be forced to sleep except by magical means.
• The next time you are reduced to 0 hit points you instead are reduced to 1 hit point and take one level of exhaustion.
• While you have only one level of exhaustion, your movement speed is increased by 10 extra feet, and you can make one weapon attack as a reaction to a creature entering your melee range.
• When you make death saving throws critical fails do not count as two failures, they only count as one. (Unless you take damage while making death saves)