Aspect of Tiamat

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

Multiple Heads. The Aspect of Tiamat can take one reaction per turn, rather than only one per round. It also has advantage on saving throws against being knocked unconscious. If it fails a saving throw against an effect that would stun a creature, one of its unspent legendary actions is spent.

Multiattack. The dragon can use its Frightful Presence or Breath Weapon. It then makes four attacks with its bite. Each bite attack has a different elemental damage: cold, acid, poison, lightning, and fire and cannot be the same type as a breath weapon used in the same round. The dragon can substitute one tail attack for one Bite.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) elemental damage. The damage is either cold, acid, poison, lightning, or fire damage.

Tail. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit:  9 (1d8 + 5) 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 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 Weapons. The dragon uses one of the following breath weapons. The dragon can use one available breath attack per turn.

Cold Breath (Recharge 5–6). The dragon exhales an icy blast in a 60-foot cone. Each creature in that area must make a DC 16 Constitution saving throw, taking 36 (8d8) cold damage on a failed save, or half as much damage on a successful one.

Acid Breath (Recharge 5–6). The dragon exhales acid in a 90-­foot line that is 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 36 (8d8) acid damage on a failed save, or half as much damage on a successful one.

Poison Breath (Recharge 5–6). The dragon exhales poisonous gas in a 60-foot cone. Each creature in that area must make a DC 16 Constitution saving throw, taking 35 (10d6) poison 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 90-­foot line that is 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 36 (8d8) lightning damage on a failed save, or half as much damage on a successful one.

Fire Breath (Recharge 5–6). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw, taking 35 (10d6) fire damage on a failed save, or half as much damage on a successful one.

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. It can only use this as a legendary action once per round.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 17 Dexterity saving throw or take 8 (1d6 + 5) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.