Depending on your level you can choose any unlocked six or even grow as you gain levels.
Tiny Sized (Levels 1-5)
Small Sized (Levels 6-10)
Medium Sized (Levels 11-15)
Large Sized (Levels 16-20)

You are Humanoid

You transform into a ghostly death cloud for the duration. You can use any remaining move you may have to move through any creatures or objects in the area. While in this ghostly state you do not trigger any attacks of opportunity nor can you take any weapon attacks. You may still take a magic attack.

Any creature you pass through takes 1d4 Necrotic damage (2d4 at level 5, 3d4 at level 11, and 4d4 at level 17). Each creature can make a DEX saving throw DC 8+CON+PB. On save they dodge out of the way 5 feet, taking no damage.

While in this form, you can move through creatures and objects but you cannot end your turn inside an occupied space.

Your speed is 30 feet with a fly speed of 15.  Your wings are made from bone and spectral smokey mist.  

You have resistance to Piercing and Slashing as determined by your Death Dragonborn ancestry.

You have Truesight with a range of 60 feet.

You have Tremorsense with a range of 30 feet.

When you take the Attack action on your turn, you can replace one of your attacks with the following:

Fantom Attack: 15-foot cone of ghostly ooze spews from your mouth taking a ranged spell attack against each creature in that area. On a hit, each target takes 2d10 Necrotic damage.  In addition, until the target uses an action to wipe off the ooze, it takes 1d4 Necrotic damage at the start of each turn and cannot regain any hit points.

This damage increases by 1d10 when you reach character levels 5 (3d10), 11 (4d10), and 17 (5d10).

You can use this attack a number of times equal to your Proficiency Bonus, and you regain all expended uses when you finish a Long Rest.

When you take the Attack action on your turn, you can replace one of your attacks with the following:

Bone Splinter Fortification: A 15-foot radius ring of splintered bones shoot up out of ground, where each creature in that area must make a Dexterity saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus). On a failed save, a creature is pushed 10 feet away and takes 1d10 Piercing damage. On a successful save, a creature is pushed only 10 feet away. This damage increases by 1d10 when you reach character levels 5 (2d10), 11 (3d10), and 17 (4d10). The area within the ring is considered difficult terrain for other creatures.

Once you use this defense you cannot use it again until you have finished a long rest.

Your character knows at least 4 languages: Common, Undercommon, Draconic plus one language you roll or choose from the Standard Languages table. Knowledge of a language means your character can communicate in it, read it, and write it.

Your class and other features might also give you languages.

The Standard Languages table lists languages that are widespread on D&D worlds. Every player character knows Common, which originated in the planar metropolis of Sigil, the hub of the multiverse. The other standard languages originated with the first members of the most prominent species in the worlds of D&D and have since spread widely.

Standard Languages

When hit with Necrotic damage you instead  absorb that as health instead, taking that number of hit points up to your maximum amount.