At 3rd level, a fighter gains the martial archetype feature. The following option is available to a fighter, in addition to those offered in the Player's Handbook: the Dark Knight.

Dark Knight

Those who follow the path of a Dark Knight often find a new outlook on life and what it means to be good or evil. A person following this path doesn't need to be evil, they just need to know that their life can offer up more strength when it is needed most. A Dark Knight uses their own life energy to empower their attacks and make sure nothing can stand up to them.

Decidedly different Outlook At 3rd level, your changed outlook on good and evil has made it difficult to for others to forcibly change the way you see things and people. You have advantage on saving throws against being charmed.

Darkness At 3rd level, you are able to manipulate your own life force to empower yourself. As a bonus action, you can sacrifice 5 hit points up to 20 hit points to add an additional 1d4 necrotic damage to your weapon attack per 5 hit points sacrificed. For every 5 hit points sacrificed an additional 1d4 necrotic damage is dealt. It is possible to sacrifice HP to a total of -5 using this feature. Doing so allows your to take your attack before falling unconscious.

Abate Darkness At 7th level, you can shrug away dark energy from an outside source. You resist necrotic damage that is not self inflicted. As a reaction you can make yourself immune to necrotic damage for 1 minute at the expense of making yourself weak to radiant damage.

See You in hell At 10th level, dealing with the energies relating to death has taught you how to punish those that take you down. When an attack puts you at 0 hit points, any enemy within a 30 foot radius of you must make a Constitution saving throw (DC = 8 + Constitution modifier + proficiency bonus). On a failure the creature takes 10d8 necrotic damage, or half as much on a successful saving throw. Once you use this feature, you can't use it again until after you finish a long rest.

Dark Nebula At 15th level, you are able to focus your dark energy into a ranged weapon against multiple enemies. As an action, you can sacrifice 20 hit points to make an attack roll against up to 3 enemies that you can see within 15 feet of you and no more than 5 feet away from each other. On a hit the target takes 5d10 necrotic damage. You can attack an extra target for every extra 10 hit points sacrificed. You can sacrifice up to -20 hit points using this feature. Doing so allows you to use the feature before falling unconscious.