Death Knight

The warrior blessed by the power of the Reaper, unaware of death, nor aware of life. Thy who take this path desire only to kill, and never to be killed

Servant of Death

When you choose this archetype at 3rd level, you learn the Spare the Dying cantrips, if you haven't already.

In addition, you automatically succeed on all of your death saving throw if you succeed on one, you can only use this feature once per long rest.

At 12th level, you can add of any additional Temporary Hit Points you have to your existing Temporary Hit Points, your Temporary Hit Points cannot exceed a number equal to half of your Maximum Hit Points (Round down).

Soul Harvest

Starting at 7th level, you can add a creature's CR x 1d8 (Min. 1) as Temporary Hit Points whenever you kill it (Round up). In addition, you gain extra Temporary Hit Points equal to your level in this class whenever you kill a creature and you still have Temporary Hit Points.

Soultaker's Protection

At 10th level, you gain resistance to radiant and necrotic damage. In addition,  when you are targeted for an attack or a spell, you can use your reaction to gain Temporary Hit Points equal to your Constitution modifier x 1d10.

This feature has a number of charges equal to your Constitution modifier  (Min. 1), you gain back all charges on a long rest.

Dead or Alive

Starting at 15th level, when your Hit Points reached 0, roll a special d20 before you fall unconscious (you do not add any bonuses to this roll). On a successful save (11 or higher),  you stay conscious and gain Hit Points equal to your Fighter level. On a failed save (10 or lower), you become unconscious and automatically fail your first death saving throw on this round.

Requiem of the Reaper

At 18th level, if an attack would reduce you to 0 hit points, but not kill you outright, you can use your reaction to force the enemy who attacked you to make a Constitution saving throw.  On a failed save, the creature takes 10d6 + 40 necrotic damage. If this damage exceeds half of the creature's maximum hit points, it is instantly killed. In addition, you regain hit points equal to the damage to have dealt. On a successful save, it take no damage, and you immediately fail one death save.

Once you use this feature, you can't do so again until you finish a long rest.

Saving Throws. This feature requires your target to make a saving throw to resist the feature's effects. The saving throw DC is calculated as follows:

Death save DC = 8 + your proficiency bonus + your Constitution modifier.