You should be dead.  You fell in battle and were by all accounts dead, but according to the fates your destiny is not yet fulfilled.  Thus, the fates brought you back under certain conditions.  You are now cursed by the fates to be the living vessel of their champion.

As their champion it is your penance to use what time you have left to fulfill some deed tied to your destiny.  If you achieve this destiny your penance will be served, and your curse lifted.  In the meantime, this curse drives your will without fear.  It does not matter what your former destiny was, the fates have cursed you and bound you to this new fate.  If you die before your penance is finished, your restless spirit will be bound to the fates forever.  Meanwhile, in your current state you are not truly alive or dead.  You are a servant and your onoly pleasure comes from serving your new masters.

Undying Fearlessness

3rd level Cursed Avenger feature

 The curse that drives you is worse than dying.  While you are bound to this curse, it takes a lot to shake your morale.  You gain advantage to all saving throws vs. fear.

Burn a Soul

3rd level Cursed Avenger feature

As their champion you are in turn serving the wills of the fates themselves.  Your purpose is to send the souls of the living to their judgement.  If you have used your Action Surge or Second Wind ability already since the last time you had a short rest, you can slay a living opponent and collect their soul for yourself.  Once you collect a soul, you can use that soul to replenish either of these abilities.

A single use of this ability can replenish one use of either your Action Surge or Second Wind.  The soul collected must be of a living entity or creature that attacked you in fair combat.  You cannot use the soul of an innocent.  This ability requires a bonus action and can be used only twice per long rest.  The action being replenished must be used before the target providing the soul is struck down. 

A soul can only replenish this ability if it has been used before the soul is collected.  It is not required that you are the one that kills the target, but it is required that you be within 30' feet at the time they are struck down.  The soul must be collected and used by the end of the following turn they were struck down.  Although both Action Surge and Second Wind can be renewed by a short rest, this ability is separate and requires a long rest to renew.  It will increase to three times at 10th level and four times at 18th level.

Imposing Curse

7th level Cursed Avenger feature

You have learned how to use your curse against others.  Your curse now drives you with a craving bloodlust for battle.  Your attacks now heal you every time you strike for critical damage against a living opponent.  Every time you score a critical hit on a living opponent you will cause them (in addition to the critical hit damage) additional necrotic damage equal to your proficiency score.  In turn, these additional hit points drained from the opponent will heal you like a vampire effect.  You cannot heal yourself above your maximum in this manner.  In addition to improving your health when you score a critical hit in combat, that opponent will feel your curse until the end of their next turn.  During this time, they will make all combat rolls and saving throws at disadvantage.  Both of these affects can only impede living opponents; thus they have no effect on undead, constructs, or other non-living opponents.

Resist Temptations

7th level Cursed Avenger feature

As your curse progresses you no longer seek human pleasures.  All foods taste the same to you, all manner of drink as well, and you no longer crave companionship.  Your only motivation is breaking your cursed existence.  Your myopic mindset allows you to resist all sleep and charm magics, and you now make saving throws vs. such effects at advantage.  You can also resist the side effects of exhaustion.

From now on when you suffer from levels of exhaustion, the side effects will be one-level weaker with the 1st level having no side effects at all.  You can no longer die from extreme exhaustion.  If you ever get to a point where you would normally die from exhaustion, you will lose consciousness and fall into a death-like sleep instead.  You will awaken from this sleep once your rest has recovered a minimum of 10% of your health.  While in this death-like sleep you will appear to be dead to all who look on you.  It requires a Medicine skill check of 20 to determine if you are still alive in this state.

Cursed Aura

10th level Cursed Avenger feature

As your curse progresses you now start to give off an unnatural aura (30’-foot radius) that makes animals and less-intelligent (intelligence 4 or less) creatures unnerved around you.  You can no longer keep a pet, familiar, animal companion, or mount, even if a multi-class feature says that you can..  Animals and non-magical beasts must make a Wisdom saving throw around you or run in fear.  The DC for this saving throw is 8, plus your Constitution modifier, plus your proficiency score. 

This aura is always on.  Any time a creature that can be affected comes within the affected area, they must make a saving throw immediately.  If they make this saving throw they cannot be affected this aura for the rest of the same day.  Creatures that fail their saving throws are able to attempts this save again every round until they overcome the affect.

At this level you can now use your Burn a Soul ability three times per long rest.

Cursed Embodiment

15th level Cursed Avenger feature

Your curse has almost fully taken over your physical self.  You have become completely obsessed with fulfilling your destiny and freeing yourself.  This is without regard that your curse has defined whom you have become more than anything else.  You are now resistant to necrotic and poison damage.  Since you cannot have a mount, the fates have given you the ability to summon your own shadowy mount.  Summoning this mount is like the use of a Find Steed spell, but the mount summoned is always a shadowy horse that never makes a sound and can run over water as if it were dry land.  You can summon this steed once per long rest.

Fear Aura

18th level Cursed Avenger feature

At this level you can focus your curse to produce a more intense version of your Curse Aura.  For one minute your aura will cause fear in all enemies of your choice within 30' feet of you. All targets will be required to make a Wisdom save or be afflicted with fear. Unlike your Cursed Aura which cannot be turned off, this ability can be used once per long rest.  The DC for this saving throw is 8, plus your Constitution modifier, plus your proficiency score.

At this level you can now use your Burn a Soul ability four times per long rest.