Adult Onyx Ghost Dragon

Amphibious. The dragon can breathe air and water.

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Ghost Stealth. While in dim light or darkness, the dragon can take the hide action as a bonus action.

Sunlight Sensitivity. While in sunlight, the dragon has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Multiattack. The dragon makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 22 (3d10 + 6) piercing damage plus 5 (1d10) necrotic damage.

Claw. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 18 (3d6 + 6) slashing damage.

Chilling Breath (Recharge 5–6). The dragon exhales chilling tendrils in a 30-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 56 (16d6) necrotic damage on a failed save, or half as much damage on a successful one. Affected creatures must then make a DC 14 Wisdom save or be under the control of the of the dragon (following the rules of the dominate person spell)

Life Drain Breath (recharge on a 6). The Dragon inhales deeply from the essence of one creature within his melee range.  The Target must make a DC 18 Wisdom saving throw or take 4d8 necrotic damage and their hit point maximum is reduced by that amount.  On a successful save they take half as much damage and do not have their hit point maximum reduced.

Spells (can each be cast once per day)

Sleep

Crown of Madness

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.

Tail Attack. The dragon makes a tail attack.

Ghost Shift. (Costs 2 Actions). The dragon can phase into a solid surface and reappear somewhere 40 ft. away on the next creatures turn.

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.