You shout "JOOR, ZAH FRUL!" At a dragon you can see. A wave of blue light shoots out in a line to the dragon, 5 feet wide. The dragon is forced to contemplate its own mortality on an extreme scale. It crashes to the ground if in flight and, cannot fly for the duration. It is the same with dragons on the ground. The dragon takes 1d8 phycic damage.
For a Wyrmling, I think you're telling them all this a little too soon and much to harsh. The dragon crashes to the ground and takes 10d6 phycic damage. It is also stunned for 1d4 turns.
For a Juvenile, I still think you're throwing all this at'em too soon. The dragon falls and takes 10d4 phycic damage. It is also stunned for 1d4 -1 turns. (Minimum 1)
Dragonborn, Sorcerors of the Draconic Bloodline, and any other dragon-related creature takes 3d6 phycic damage if they try to cast this spell and the damage to the dragon is halved, and they are not stunned and can get back in the air as an action on their turn. Drakewarden rangers do not take damage unless their Drake is in the area of effect, which in that case they both take 2d4 phycic damage.
THIS IS A THU'UMIN' SHOUT NOT A SPELL!!!!!!!!!