Legendary Resistance (2/day). If the Abyssal Dragon fails a saving throw it can choose to succeed instead.

The Dragon's Skin. The dragons skin has the same properties as the cloak of displacement, if the dragon dies the skin loses this property. It projects an illusion that makes you appear to be standing in a place near your actual location, causing any creature to have disadvantage on attack rolls against you. If you take damage, the property ceases to function until the start of your next turn. 

Multiattack. The abyssal dragon makes four attacks: one with its bite, one with its claw, and two with it's tail and can use Abyssal Breath as a reaction.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 51 (10d8 + 7) piercing damage

Claw. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 27 (6d6 + 7) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 25 ft., one target. Hit: 33 (6d8 + 7) bludgeoning damage and the target must succeed on a DC 22 Strength saving throw or be pushed up to 15 feet in a straight line away from the abyssal dragon and knocked prone.

Abyssal Breath (Recharge 5-6). The abyssal dragon exhales fire and acid in a 60-foot cone. Each creature in that area must make a DC 22 Dexterity saving throw, taking 66 (12d10) acid damage and 66 (12d10) fire damage on a failed save, or half as much damage on a success. 

The abyssal 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 abyssal dragon regains spent legendary actions at the start of its turn.

Attack. The abyssal dragon makes one claw attack or one tail attack.
Detect. The abyssal dragon makes a Wisdom (Perception) check.
Insanity (Costs 2 Actions). The abyssal dragon targets one creature it can see within 30 feet of it. The target must succeed on a DC 20 Wisdom saving throw or be haunted by images of shadows and demons. When a creature haunted by these images makes an attack roll, it repeats its saving throw; on a failed save, it must target the closest creature it can see within the range of the attack. If multiple creatures are equally close, determine the target randomly. On a successful save, the effect ends for it. Otherwise, the effect lasts for 1 minute or until the creature benefits from a lesser restoration spell or similar magic.
Self Destruction (Costs 2 Actions). The abyssal dragon uses its Abyssal Breath, even if it isn't recharged. It then takes 66 (6d10) necrotic damage. This damage cannot be reduced or avoided.

Enter the description for how mythic actions work for your monster here.

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.