Cernunnos

Curse-Decayed Flesh: Cernunnos start combat with a deadly curse on it's body. This curse grants it resistance to all attacks besides radiant. If hit by radiant damage it doesn't gain it's benefits form Cursed Layer until it's next turn and it losses it's resistances gained with this ability until it's next turn. Also while this is active this creature can not be affected by status ailments.

Curse Layer: If Cernunnos takes damage from a creature his curse spreads. The creature must make a Con Save (DC 25) or be cursed. This curse hurts the target/s and heals Cernunnos. The amount equals TO 5D6 and it happens at the start of Cernunnos's turn. This curse can only be removed with greater restoration , the Wish spell, Remove Curse cast at the 5th level or, if Cernunnos is  defeated.

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

Multi-attack. The Cernunnos can use its Frightful Presence. It then makes three attacks 

Cursed Claws. Melee Weapon Attack: +18 to hit, reach 30 ft., one target. Hit: 19 (2d10 + 8) [necrotic] damage. Then if this attack hit the target they must make a Con Save (DC 20) or be Slowed. The creature must make the save at the end of their turns to break free from the effects.

Cursed Ray. Ranged Weapon Attack: +18 to hit, range 120/600 ft., one target. Hit: # (4d6 +8) [Acid] damage. Then if the target is hit they must make a Con Save (DC 20) and if they fail they can not be healed by healing effect unless they pass the save at the end of their turn.

Frightful Presence. Each creature of the Cernunnos's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 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 Presence for the next 24 hours.

Cursed Annihilation (Recharge 5–6). Cernunnos exhales acid in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC 22 Dexterity saving throw, taking 67 (15d8) acid damage on a failed save, or half as much damage on a successful one. Creatures that fail the save also must then make a Con Save (DC 20) or become paralyzed and must make the con save at the start of their turns to be freed from the effect.

"Please don't go". As a reaction if Cernunnos is attacked by a creature cursed by Curse Layer they can make that creature make a Charisma Save (DC 20). If the creature fails the save the effects of Curse Layer active immediately and that target is Stunned for one round.  

Cernunnos 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. Cernunnos regains spent legendary actions at the start of its turn.

Cursed Claw (Costs 1 Actions). Cernunnos can use it's Cursed Claw.

Move (Costs 2 Actions). Cernunnos can move half it's movement speed.

Cursed Aura Wave (Costs 3 Actions). Cernunnos can make creatures up to 120ft around it to make a Wisdom Save (DC 25) taking 40 (8D10+8) necrotic damage, half if passed the save. Creatures that failed the save become cursed as if by Curse Layer and if the creature is already cursed then the curse evolves. The curse now deals 10D6 damage for those creatures.