Chromatus

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

Multiheaded. Chromatus gains advantage on perception checks and cannot be flanked.

Multiattack. The dragon can use its Frightful Presence. It then makes five attacks: one with each head to bite. It may instead replace up to 2 bites with a breath weapon. 

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 7 (2d6) poison 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 16 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.

Breath Weapon (Recharge 5). The dragon exhales from one of its heads. Each creature in that area must make a DC 18 Constitution saving throw, taking 56 (16d6) damage on a failed save, or half as much damage on a successful one. Each breath can only be used once every 5 turns 

• Fire 30ft area within 80ft
• Ice 30ft cone
• Lightning 80ft beam
• Poison 45ft cone
• Acid 30ft area within 60ft

An enemy that would be hit by 2 breath weapon attacks on the same turn has disadvantage on both saving throws.

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.

Lunge Attack. The dragon slams forwards, moving up to 20ft, all creatures within the area it slams must succeed on a DC19 Dexterity saving throw or are knocked prone take 13 (2d6 + 6) , and back to an unoccupied square.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Ferocious Breath (Costs 3 Actions). The dragon uses all breath weapons it did not use on its turn, it then may not use any breath weapons on its next turn