When you are not possessing anything and the start of each turn during combat or once every hour outside of combat, you take damage equal to your proficiency modifier. 

You gain +2 to your dexterity and strength, but you have disadvantage on Nature, Animal Handling, Medicine, and Religion checks.

At 15th level, can use the abilities of the creature you are possessing alongside your own.

When you are not possessing a body, you float slightly above the ground (or whatever surface you are on) leaving you unaffected by traps hidden in or on the ground.

As an action or free action, you can unpossess or possess another willing creatures body (medium or small). If the creature is unwilling, you can do a contested Wisdom roll. If you win, you can take over the creatures body, if you lose, you do not possess the creatures body. When possessing another creatures body, you use your own stat block and abilities (Until a certain level, that is).
While you are possessing a body, any benefits it gains from a long or short rest, also applies to you. Also, when the creature you are possessing has their hitpoints reach 0 and are stable, you too are knock unconscious. 

At level 20, you can take the life force of the creature you are possessing for your own, killing it. You regain your original form and it's benefits. You lose you 10 flying speed but you are no longer affected by the effects of "Unpossessed."