Voidwalker

Your own life force is sapped from you, though you may do the same for others whom deserve such a fate as you. Fighting by using those dark powers are what fuel your own abilities. Wise actions have few consequences.

They Who Walk

At 3rd level, a Voidwalker is born, and has their maximum health reduced by 20% (Rounded up), and every further level after gaining the subclass, when you gain max hp from hit die, reduce it by 2. At 7th level, the effect becomes 30%. You may exchange these out at dawn, during any kind of rest. You may hold up to 3 d4's at a time. After which you gain no more until one is used. You may use multiple in one turn, capping at 2. You may not use the same one twice in a turn.

When damaging a creature, once every second attack, you may add a single d4 of necrotic damage, which you may use for the following. {Choose up to 3}

Dance of Death: Heal for the amount of damage dealt by the d4 + your constitution modifier. If the damage is resisted, bypass it for only the healing. The damage will be reduced, yet the healing may remain the same. This damage is applied after attacking, and as such cannot be critical.

Maim: Reduce a creatures speed by 5 until its next long rest. (Up to 15 to each type of speed.)

Blitz: Make another attack, which uses your weapons accuracy die, but the damage becomes the d4.

Siphon: If the target's last spell is Equal to your Fighter level halved (rounded down) or less, you may cast it at its lowest level, to a maximum of 4th level spells. (Enemy Casts Cure wounds 3rd level, and you are 6th level Fighter, you may cast the spell at 1st level.) {7th level required.}

Reckless Battle: You may double the damage of the d4, at the cost of taking half of whatever damage is dealt. {7th level required.}

Dauntless Defense: Passively, as you gain d4's, when wearing light or medium armor, add the number of dice to your armor class.

Sightseer

Gain Proficiency in Perception, and gain advantage on rolls to spot minor details on things even up to 100 feet away.

Overheal

At 10th level, you defy the laws of nature, and may heal over your maximum health, up to 15. This effect is granted as temporary health, and only lasts for a minute. Temporary hit points will only stack if you are the one who applies them.

Additionally, when you recover health in any way, reduce the next damage you take by your constitution modifier.

Toeing the Line

By allowing the Void to gain a tighter grip upon your soul, you allow yourself to take more of the entities power.

This effect may be activated once per long rest lasts 10 minutes, and does the following:

Take force damage equal to half of your current health, this damage is unmitigated by any source. You cannot heal above the amount of health you are left with until the effect is over. Next, gain resistance to any damage type of your choosing, as the void wraps itself around you. Furthermore, self healing or temporary health you receive doubles.

You gain the capacity for an additional d4 during the effect, which if held after the effect goes away, disappears into the Void's power.

Void Master

You've beaten the Void's total grip on you, but there was great cost.

Your maximum health remains as it were before.

But:

Your "Second Wind" trait has 3 charges, of which can be expended as bonus actions to provide its normal effect, or to apply double the effect of your d4's.

Any effect that attempts to gain access on your mind fails on a contested wisdom save with its spell DC or its own wisdom.

Aberrations take twice as much damage from your d4's, and cannot resist the damage.