Incorporeal Nature

A number of times equal to your proficiency bonus you become Incorporeal allowing you to pass through creatures and objects as if they were difficult terrain, if you end your turn in an object/creature you take 1d10 force damage. 

Every time you use this feature you must roll 1d6 on an odd number you stay in the material plane and on even you pass into the ethereal plane 

Ethereal - Gain a flying speed and 10ft Blind sight

Material - Gain walking speed and 60ft Dark vision 

Due to the damned nature of your soul you cannot move willingly move more than 100ft from what you are bound to.

If separated for more than one day your soul loses the link to the material plane and is lost in the ethereal plane.

Possession 

Two times a day you can choose one humanoid that the ghost can see within 5 feet of it must succeed on a DC 14 Charisma saving throw or be possessed, automatic fail if dead or unconscious, the Eidolon disappears, becomes untargetable and the target loses control of their body.

The possession lasts until the body drops to 0 hit points, the ghost ends it as a bonus action, or the ghost is turned or forced out. When the possession ends, the ghost reappears in an unoccupied space within 5 feet of the body. The target is immune to this ghost's Possession for 24 hours after succeeding on the saving throw or after the possession ends.

Fright

While in this form you gain the ability to fright

As a bonus action you attempt to use the possessed body to frighten other creatures in a 10ft radius, the must succeed a wisdom saving throw DC 14 or become frightened of you

Due to the undead nature of your being you no longer need to sleep, eat, drink or breathe.