You are a master of vibrations.

• As a bonus action, you choose an area of stone or mud that you can see that fits within a 10-foot cube that is still within 5 feet of you, and choose one of the following effects.
  Nonmagical Earth of any sort in the area becomes an equal volume of thick, flowing quicksand that remains until the end of your next turn.
  The ground in the abilities area becomes full of vibrations, making it a strong quicksand that creatures can sink into. Each foot that a creature moves through the quicksand costs 4 feet of movement, and any creature on the ground when you cast the spell must make a Strength saving throw equal to your spell saving throw. A creature must also make the saving throw when it moves into the area for the first time on a turn or ends its turn there. On a failed save, a creature sinks into the quicksand and is restrained, though it can use an action to end the restrained condition on itself by pulling itself free of the mud.

You can use this ability equal to your proficiency modifier.

Increase one score by 2 and another score by 1

Young dragonborn grow quickly. They walk hours after hatching, attain the size and development of a 10-year-old human child by the age of 3, and reach adulthood by 15. They live to be around 80.

Dragonborn are taller and heavier than humans, standing well over 6 feet tall and averaging almost 250 pounds. Your size is Medium.
In your shelled form you have more height and weight, Multiplying your weight by three.

Your base walking speed is 30 feet.
Your burrowing speed is 15 ft.
In your true form, your burrowing speed is equal to your walking speed

You gain a breath weapon and damage resistance with your chosen dragon type. (Thunder)

You can use your action to exhale destructive sonic energy in a 5 by 60 ft line. When you use your breath weapon, each creature in the area of the exhalation must make a saving throw, the type of which is determined by your draconic ancestry. The DC for this saving throw equals 8 + your Wisdom modifier + your proficiency bonus. A creature takes 5d6 damage on a failed save, and half as much damage on a successful one. The damage increases to 5d6 at 6th level, 6d6 at 11th level, and 8d6 at 16th level. After you use your breath weapon, you can’t use it again until you complete a short or long rest. Your breath weapon can only be used in your True Form.

When you use your breath weapon while underground, the closest creature you target is also subjected to your Mystic Vibrations without using a charge. 

You have resistance to Thunder damage

You can speak, read, and write Common, Primordial, and Draconic. Draconic is thought to be one of the oldest languages and is often used in the study of magic. The language sounds harsh to most other creatures and includes numerous hard consonants and sibilants.

At the end of a long rest, you gain temporary hp equal to 5 times your proficiency modifier.
While you have this temporary hp you gain certain qualities.

• You gain resistance to Slashing and Piercing.
• Your Thunder resistance becomes Thunder immunity.
• You are vulnerable to Bludgeoning damage.
• Your AC increases by 2
• You can cast Shield as a reaction once.

These Qualities last until you run out of Temporary HP, or you cast the shield spell as a reaction.
Other sources of Temporary HP can stack with this Hp, But the Temporary HP from this ability does not stack with any other source.
Finally, when you lose all of this Temp hp, your Earthshell crumbles off and your burrow speed becomes equal to your walking speed.