Draconic Presence

Touch a willing creature. If not willing, the creature must make a wisdom saving throw or be transformed into a colossal dragon, the color of such, is chosen by you.

The target then takes the statistics of an ancient dragon of that coloration, but they keep their mental statistics, and they, in the draconic form, the target's hit points is ten times as much as that of the standard ancient dragon. The flight and walking speed is also multiplied by ten. 

The resulting dragon form is 20 times the size of the standard ancient dragon, being of Colossal scale instead of Gargantuan.

If the target is unwilling, and succeeds on their save, their form is transformed into a draconic version of their own race. On a fail, they turn into a dragon of the spell's description.