Ghost Knight

Ages ago, some knights and warriors began to commune with beings from the world of ghosts and spirits, studying their nature in order to replicate them on the battlefield. This tradition is known for its close association with death, the spiritual realms, and the feverish dedication with which followers study the undead, learning and understanding all that there is to know about them.

Known as Ghost Knights, these fighters develop abilities to see and enter the spirit world, imbue their weapons with ghostly power, and eventually can become incorporeal themselves.

Ethereal Strikes

Starting at 3rd level, you're able to imbue your weapon strikes with ethereal energy. As a bonus action, you can cause a weapon you are holding to emit ghostly flames for 1 minute. For the duration, your attacks with this weapon ignore all cover except that which is provided by an object made of magical force (such as from the wall of force spell), and all damage dealt by the weapon becomes force damage. Additionally, you can target creatures on the Ethereal Plane within the weapon's range or reach, provided you can see them.

Once you have used this feature, you cannot do so again until you finish a short or long rest.

Undead Knowledge

At 3rd level, your studies into the undead and spirits has granted you knowledge about them. You have advantage on Intelligence checks made to recall information about undead.

Spectral Vision

Beginning at 3rd level, you have acquired a special ability for detecting the presence of spirits. As a bonus action, you can cause your eyes to become pale and emit ethereal smoke, allowing you to see up to 30 feet into the Ethereal Plane while you are on the Material Plane, and vice versa for up to 1 hour. You can use this feature twice per short or long rest. 

Return from the Grave

By 7th level, your connection to death allows you to return from deadly attacks. When you are reduced to 0 hit points but are not killed outright, you can drop to a number of hit points equal to half of your fighter level.
Once you have used this feature, you cannot do so again until you finish a short or long rest.

Ghost Walking

At 10th level, your body has developed ghostly qualities. You can now move through creatures as if they were difficult terrain. If you end your turn in a space occupied by a creature, you take 5 force damage, and you are immediately pulled to the nearest unoccupied space, taking 5 force damage for each 5 feet that you move.

Haunting Attunement

At 15th level, your time spent amongst spirits and undead has toughened your body and resolve. You gain resistance to cold and necrotic damage, and you become immune to the frightened condition.

Spectre Knight

By 18th level, you can truly move like a ghost, you gain a flying speed equal to your walking speed.