You have obtained the power of ghosts and apparitions, and you use it to skulk and fight like a deadly shadow, vanishing and reappearing to spook your foes, passing through walls, and striking through your foes' armor. You might have gained this ability through a dark ritual, or perhaps you were born upon desecrated ground and it influenced you in unseen ways. You might have even developed these powers after dying and being brought back to life by powerful necromancy.

Spectral Skulk

Starting at 3rd level, you can phase your body into and out of the Ethereal Plane in a limited way, much like a ghost can. You can use a bonus action on your turn to teleport up to 30 feet to an unoccupied space that you can see, and then you can choose to Hide as part of that bonus action. If you don't Hide, each creature within 5 feet of you when you appear is startled until the end of that turn. You can use your Sneak Attack against a startled target even if you don't have advantage on the attack roll, but not if you have disadvantage on it.

In addition, you can use your action to reach one of your arms through solid material within 5 feet of you as you phase your arm into the Ethereal Plane. You can interact with an object as part of this action, and when you do so, one arm of your choice, along with anything you are wearing on that arm, can ignore the presence of solid objects of your choice, even solid walls, passing harmlessly through them with your arm. You can still grab and hold objects that you do not choose to pass through, but you can only carry them through solid material if what you are carrying in that arm weighs less than half of your Strength score in pounds.

You can reach through most materials using this action, but 2 feet of rock, 2 inches of any metal other than lead, or a thin sheet of lead blocks you. You can't use this action to make an attack.

You can only use this bonus action and this action a number of times each equal to your proficiency bonus. You regain all expended uses for both portions of this feature when you finish a long rest.

Ghost Assassin

At 9th level, you gain the ability to wreathe your weapons in spectral energy. During your turns, your weapon attacks count as magical for the purpose of overcoming damage resistances and immunities.

In addition, you can use a bonus action to see with ghostly sight. For the next 1 minute, you can see other creatures that are invisible as if they were visible, and you can see into the Ethereal Plane. Ethereal creatures and objects appear ghostly and translucent.

Once you use this feature to see with ghostly sight, you can't do so again until you finish a short or long rest.

Shadow Form

Starting at 13th level, you can use a bonus action on your turn to transform into a ghostly apparition for 1 minute. While in this form, you have the Incorporeal Movement trait, attackers have disadvantage on opportunity attack rolls against you, you have resistance to damage dealt by opportunity attacks, you gain a flying speed equal to half your speed, and you can hover.

Incorporeal Movement. You can move through other creatures and objects as if they were difficult terrain. When you take radiant damage, you can't move through objects until the end of your next turn. If you end your turn or take radiant damage while inside an object, you take 2d10 force damage and you are pushed into the nearest unoccupied space in a random direction.

Once you have used this feature, you can't use it again until you finish a short or long rest.

Penetrate Armor

At 17th level, you learn how to use your powers of phasing to pass directly through your opponent's defenses. When you make a weapon attack against a creature, you can choose to phase your attack so that you ignore armor.

Instead of you rolling for the attack, your target must succeed on a Dexterity saving throw or else be hit by your attack automatically. The DC for this saving throw is equal to 8 + Dexterity modifier + your proficiency bonus. The target has disadvantage on the saving throw if you would have had advantage on the attack roll, and it has advantage if you would have had disadvantage on the attack roll. If the target rolls a 1 or a 2 on the d20 for its saving throw, the attack critically hits.