Etherblade

The knife is a tool - a tool which may pierce the veil between realms. You have learned to use your skills to pierce the ethereal plane and draw its powers to infuse them into your own. This does not come without cost, however. The transition between planes renders your physical form moot. To harvest, there is an opportunity cost. To strike, a cost of energy. To live, a cost of blood.

Ethereal Battery

At 3rd Level, your blade becomes a tool to store energy drawn from the ethereal plane. When you hit with a weapon attack, you may forgo dealing damage and instead gain 1 Ethereal Charge. You may have no more than three ethereal charges at one time, and each ethereal charge dissipates after one minute.

When you hit a target with your Sneak Attack, you may expend any number of Ethereal Charges to deal an additional 1d6 damage per charge spent. Alternatively, you may expend 2 Ethereal Charges to use any Cunning Action without expending a bonus action. You may not gain Ethereal Charges after dealing damage with a Sneak Attack, and you may not use Charges to use Cunning Actions more than once per turn.

Kinetic Overload

At 9th Level, you learn how to store additional energy into your blade and use it for additional purposes. You may now store up to 5 Ethereal Charges at once. When hitting an enemy with Sneak Attack, you may do any of the following:

- Distract: You may expend one charge to give the target disadvantage on its next attack roll.

- Disarm: You may expend two charges to force the target to make a DC 17 Strength Saving Throw. On a failure, the creature drops its weapon and must use an action to pick it back up.

- Destroy: You may expend two charges to cause all extra damage dealt by expending charges to spread to all hostile creatures within a 20-foot radius.

Space Walk

At 13th Level, you learn how to use your stored energy to shift yourself out of existence. As an action on your turn, you may expend 3 Ethereal Charges to become Ethereal until the end of your next turn. While Ethereal, you are invisible to all creatures and cannot be the target of any attacks. You may walk through solid objects, though if you end your turn in one, you take 2d6 force damage, are pushed to the nearest unoccupied space, and you return to the material plane, ending the Ethereal condition. While Ethereal, you gain advantage on all attack rolls, and your attacks deal an extra 2d6 psychic damage. 

This condition ends early if you take or deal damage. 

Befuddling Strikes

At 17th Level, you have mastered your craft. You may now store up to 7 Ethereal Charges at a time, and when hitting a creature with Sneak Attack, you may expend an additional 3 Ethereal Charges to force the creature to make a DC 18 Constitution Saving Throw or become stunned until the start of your next turn. You may expend any number of Ethereal Charges to raise the DC by 2, to a maximum of 22. Charges expended this way do not count towards additional damage dealt.