Asmodeus The Terrible

Legendary Resistance (6/Day). If the dragon fails a saving throw, it can choose to succeed instead. 

Disintegration:  The horror rears its head back as lightning spark down the monsters body and fire erupts from its scales. It will take one turn of combat to charge this devastating attack, combining fire and lightning dealing 15d10 Fire Damage and 15d10 Lightning Damage. Critical Hitting on a 18-20

Molten Aura: The Dragon can enter an enraged state if it takes over 50 points of HP in one round of combat it can enter a state of rage. The dragons hide burns red hot and everything around it begins to warp and melt. Anyone in the radius takes 6d6 Fire Damage every turn of combat 

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

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 14 (4d6) fire damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

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

Fire Breath (Recharge 5–6). The dragon exhales fire in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 193 (34d6) fire damage on a failed save, or half as much damage on a successful one.

Lightning Breath (Recharge 5-6). The Dragon exhales lightning in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 88 (21d10) lightning damage on a failed save, or half as much damage on a successful one.

Daunting Roar. As a bonus action, the dragon lets out a horrifying roar. All creatures in a 130 radius must make a Wisdom Saving throw. Failure of the save results in being frightened until you leave the dungeon the dragon resides in. Succeeding the save causes you to be frightened for 1 hour or until you remake the save. Save DC 19

Heated Body. A creature that touches the dragon or hits it with a melee attack while within 5 feet of it takes  (8d6) fire damage.

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.

Unfurl . The Dragon opens its gargantuan wings in the dungeon, prompting a wisdom saving throw of 21. All creatures that fail the save are frightened of the dragon until the round of combat ends. 

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 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Mythic Encounter:  The Dragon always is at the end of initiative.