Cursed Dragon Cerberus

Legendary Resistance (3/day). If Cursed Dragon Cerberus fails a saving throw, it can choose to succeed instead.

Keen Hearing and Smell. Cursed Dragon Cerberus has advantage on Wisdom (Perception) checks that rely on hearing or smell. 

Multiple Heads. Cursed Dragon Cerberus has three heads. While it has more than one head, the Dragon Cerberus has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.
Whenever Cerberus takes 60 or more damage in a single turn, one of its heads dies. If all its heads die, Cerberus dies.
At the end of its turn, any heads that died since its last turn regenerate and Cerberus heals 20 hit points.

Reactive Heads. For each head Cerberus has beyond one, it gets an extra reaction that can be used only for opportunity attacks. 

Wakeful. While Cursed Dragon Cerberus sleeps, at least one of its heads is awake.

Multiattack. Cerberus makes as many bite attacks as it has heads and one claw attack or one tail attack.

Bite. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 20 (3d8 + 7) piercing damage plus 14 (4d6) lightning damage. 

Claw. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 16 (2d8 + 7) slashing damage.

Tail.  Melee Weapon Attack :+7 to hit, reach 5 ft, one target hit 8(2d10+7) bludgeoning damage

Frightful Presence. Each creature of the dragon's choice that is with in 120 feet  of the dragon and aware of it must succeed on a DC 18 wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of it 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 presence for the next 24 hours 

Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.

Lightning breath  the dragon exhales lighting in a 80 ft cone . Each creature in it most make a dc 11 dexterity saving throw, taking 67 (15d8) lightning damage on a failed save or half as much on a successful one.

Weakening Breath. The Cursed Dragon Cerberus exhales gas in a 30-foot cone. Each creature in that area must succeed on a DC 17 Strength saving throw or have disadvantage on Strength-based attack rolls, Strength checks, and Strength saving throws 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.

Screeching Howl. Cursed Dragon Cerberus can use its reaction Screeching Howl only if one of its 3 heads can see a target in a line in front of it. The Cerberus Screeches with a forceful that pushes the creature or target 10 to 30 feet away into a nearby object. Creature must succeed a DC 19 Dexterity throw or take 17 (2d6 + 10) Force damage. on a successful save this reaction does nothing.

Cursed Dragon Cerberus can take 4 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 Cursed Dragon Cerberus regains spent legendary actions at the start of its turn. 

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

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 12 (2d6 + 5) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Pounce (Costs 2 Actions). Cerberus can jump on top of its target. Each creature within 15 feet must succeed a DC 25 Strength saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The pounced target is considered grappled and prone.