Amphibious. The dragon can breathe air, water and vacuum.
Legendary Resistance (3/Day). the dragon can take the attack on him and gain 2d10 hp from there
Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) acid damage.
Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.
nacrotice Breath. The dragon exhales acid in a 60-foot line that is 5 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 54 (12d8) acid damage on a failed save, or half as much damage on a successful one.
change shape:
the dragon can change elements, making it resistant to other things, the choice is: fire, cold, lightning, acid and psychic. the dragon can do this 3 times a day
memory beam:
if the dragon is attacked it can choose to use the memory beam if it does this all creatures in a 30 feet circle will have a painful flashback and take 6d6 damage this can use the dragon 4 times 6[roll]6d6+2[/roll]
The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.
black hole. the dragon can create a black hole that for 2 rounds will keep all creatures 10 feet from hitting prone
Tail Attack. The dragon makes a tail attack.
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Lair and Lair Actions
if everything is destroyed then the dragon can change the environment however he wants