Way of the Spectre.

Witches of the Way of the Spectre go through harrowing training to acquire their abilities. After years of rigorous schooling and instruction of both their minds and bodies, a covenant ceremony takes place with their mentor in which the pupils experience death itself. The disciples effectively die, sent to the plane where spirits lie for a short moment, before being brought back to life by witch healers. Many witches in training do not survive this ceremony — or, at least, not entirely. They come back as empty husks, devoid of emotion and personality, part of their soul forever lost among the spirits. Those who do take this ultimate risk and make it back gain the key to this specific art of witchcraft: they can use their body itself to channel spirits. Their expertise lies in weakening their foes with attacks charged with spiritual energy, summoning spirits with each blow and driving them into the body of their target to siphon the target’s life energy.

Witchmark.

A tell-tale sign of a witch following this tradition is their unusual witchmark. Rather than resembling a tattoo or a specific marking, the witchmark of these witches is their hands themselves. They twist and change due to the prolonged contact with spirits.

Level 3 Features:

Weakening Blows.

You summon spirits that encircle your fists or weapons, and you use them to weaken the bodies of those receiving your blows. When you successfully hit a creature with an attack, you can spend 1 ki point to force the creature to make a Constitution saving throw. On a failure, the creature’s vitality is weakened as the spiritual energy from your blows courses through their bodies. The creature takes one level of exhaustion. You can spend 1 ki point per attack to repeat this effect, but you cannot inflict more than three levels of exhaustion to one creature. At the end of each of the affected creature’s turns, they lose one level of exhaustion. For instance, if you inflict three levels of exhaustion on a creature, at the end of the creature’s following turn they will drop to two levels of exhaustion; the following round it will drop to one; and the following round it will lose all exhaustion.

Level 6 Feature:

Death Spiral.

You learn to manipulate the spiritual energy you have injected with your blows within a creature’s body, further damaging the target from the inside. Whenever you hit an enemy with at least one level of exhaustion with an unarmed strike, you deal one additional damage die of the attack you make against that creature for each level of exhaustion that creature is under.

Level 11 Feature:

Siphoned Life.

By injecting spirits into the body of a foe, you siphon out their energy to replenish your own. Whenever you use Weakening Blows to give a creature a level of exhaustion, you are healed for an amount equal to the damage roll of that attack. You can use this ability a number of times equal to your Wisdom modifier. You regain all uses of this ability after a long rest.

Level 17 Feature:

Siphoned Essence.

You have gained mastery over driving spirits into your foes’ bodies, and are now able to affect their very essence. Select an ability score you want to damage other than Constitution. When you make an unarmed strike, you can use 5 ki points to force your target to make a saving throw. The saving throw will correspond to the ability score you are targeting. If the creature isn’t surprised, they make this saving throw with advantage.
If your target fails the saving throw, roll your martial arts damage die. The ability score you chose is lowered for your target by half of the martial arts roll. If the creature’s ability score reaches 0, they automatically fail any ability check or saving throw using that ability score.
You must maintain concentration on this ability as if it were a spell, although it cannot be ended by spells like Dispel Magic. The effects of this ability end after 1 minute, when you or the creature’s hit points reach 0, or you lose concentration. You can use this feature a number of times equal to your Wisdom modifier. Whether your target succeeds or fails their saving throw, you cannot use this ability again on the same creature until you finish a long rest. You regain all uses of this ability after a long rest.