Corrupted Dragon

Amphibious. The dragon can breathe air and water.

Soul Seizer. If the Dragon kills a creature its soul is instantly consumed and cannot be revived by any means. Tendrils of dark, viscous slime then whip off the dragon. All creatures within 20 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 21 (4d6 + 5) necrotic damage. The dragon then heals for the amount of damage dealt.

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

Multiattack. The dragon can use its Frightful Visage. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8) piercing damage.

Claw. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.

Frightful Visage. Due to the aberrant influence of this dragon, all who look upon the dragon are filled with horror. Each creature who has laid eyes upon the dragon within 120 feet must succeed on a DC 21 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Visage for the next 24 hours.

Breath Weapons (Recharge 5–6). The dragon exhales its breath in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 52 (15d6) damage on a failed save, or half as much damage on a successful one. (Damage type is dependent on dragon type)

Corrupt Mutation: Upon taking enough damage to lose half of the dragons hit points the dragon slips further into the clutches of the aberrant infection, its body begins to mutate and change.

When the dragon loses half its hit points roll a d6 and see the list below:

(1) Twin Heads. The dragon's head splits down the middle, pulling itself apart to have two heads. Each head can now use breath weapons and make individual bite attacks.

(2) Tentacle Tails. The dragon's tail tears itself apart into two tentacles. Each tentacle can use a tail attack, or grapple using the dragons Strength modifier. 

(3) Eyes of the Beholder. The eyes of the dragon become able to project a 60 ft anti-magic cone. Creatures inside the cone are under the effect of the antimagic field spell.

(4) Boney Spikes. Scales and bones snap and pierce the hide of the dragon. When the creature is hit with a melee weapon, the attacker must make a Dexterity saving throw or take 7 (2d6) Piercing damage.

(5) Slimy Scales. The dragon's scales begin to secrete a think mucus. This mucus increases the dragon's AC by 2 as well as giving it resistance to all damage types except Psychic and Necrotic.

(6) Winged Claws. The dragon's wings snap and splinter. The bones that once held the winged filament now stretch into another set of claws. The dragon can now make two more claw attacks.

These changes work with the dragon's base actions.

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.

Detect. The dragon makes a Wisdom (Perception) check.

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 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.