You are a Elemental

You are immune to poison damage, resistant to the same damage type as your element, and vulnerable to fire and bludgeoning damage.

Your size is small

Your speed depends on the current race your are controlling. You gain the base speed of that race.

The ability to sense the world around you. Grants the sense of sight, hearing, and feeling to the body until further growth. 

An effect that only takes place after death, when your character dies your crystal shatters into pieces. The largest of those pieces can be salvaged to reforge the crystone at the cost of losing levels. This effect sets back to your character and hinders your abilities. 

As you level up, you will level up twice until you are caught up to the appropriate level.

As an action you are able to launch yourself out of your corpse in any direction up to 15ft. This will leave your core vulnerable but may be handy for getting out of rough situations. If you land on another corpse you may chose to enter the corpse and use it as a new puppet gaining the racial features of the corpses race. 

At 7th level, you can use this ability as a bonus action.

At 10th level, Your Magic Sense becomes refined enough to manipulate the magic coursing through other creatures.

As an action If you come into contact with another living creature, you can choose to take control of that creature. The creature must make a Intelligence saving throw equal to your spell save DC. On a  successful save, nothing happens and you fall to the ground at the target feet. On a failed saving throw, the creatures magical capabilities are syphoned by you as you merge with its body gaining control of the creature for 3 rounds.

After 3 rounds the creature must use its reaction at the beginning of its next turn to launch your core 15 ft back into your original corpse . If the original corpse is somehow destroyed or out of range, the creature must launch you as far as it can in the direction of the original corpse.

At 15 level, replace Hostile Takeover with Indomitable  Elemental.

After understanding what forms are and how to control one, you learn how to create your own. Becoming a force of nature only the dragons could understand.

Before a long rest you can chose to forge an entirely new form matching your element, or reforge your form  by spending some time to piece your form back together.  This allows you to form bodies that may be identical some days and different another.

Indomitable Wave

This new form allows you to emit a terrifying wave of elemental energy within a 15ft cone. Creatures in the cone must make a Charisma saving throw equal to your spell save DC. On a successful saving throw, the creature takes 2d6 plus proficiency modifier amount of damage. On a failed saving throw, the creature will take 2d8 plus proficiency modifier amount of damage and is frightened until the end of its next turn. the damage type matches your crystones element.

The damage increases by 1d8 for every 2 level ups