Dragon Transformation

At Sorcerer levels 15, 17 and 20, If you purposefully draw your own blood, the body of a dragon will form around you; the nervous system, bones and flesh forming one after another out of thin air. Once the full body, or just the muscles, are formed, you have full control over the motor functions of the dragon as well as its magical capabilities. The formation of the body is initiated with a bolt of lightning, and then a small hurricane consisting of the element of whichever Draconic Ancestor is within your blood. If the dragon body is to receive a large amount of damage, or you wish to exit early, you will be released out of a fissure which forms on the body's back; scalding hot steam will erupt out of said fissure, so make sure to warn any companions before hand, but, the steam will not harm you despite its temperature. Upon fully exiting the dragon body (all limbs are removed) it will begin to rapidly deteriorate, a plethora of hot steam will be released from the dragon body which could be used as a smoke-screen of sorts to covertly escape.

Using the dragon body is an arduous task and once you leave it, you receive 3 points of exhaustion. If you are in the dragon body for an extensive amount of time or take a large amount of damage, you must pass a Constitution saving throw, default of 15, to determine whether you emerge unconscious or not. (The save DC may be changed by the DM).

The age and power of the dragon you form is dependant on your level: 15 -> Young Dragon, 17 -> Adult Dragon, 20 -> Ancient Dragon

At level 20, you may create a Colossal Dragon body, however, this is only a one time use and you must pass a DC 18 Constitution save in order to not die of exhaustion once done. The Colossal Dragon is the same size as, and has the same health as, the Tarrasque but the same abilities as an Ancient Dragon. Transformation into the Colossal Dragon requires the sacrifice of one of your limbs to succeed, the limb sacrificed is determined by the DM and is kept unknown to the players until after exiting the dragon. (The sacrifice may only remove one of either arms or one of either legs unless the player is killed by failing the saving throw, if they fail any portion of the body may be sacrificed). 

Dragon Transformation consumes an 8th level spell slot( or 9th if Adult, Ancient or Colossal); if that slot can be returned, the spell may not be used again until after a long rest.