Discorporation. When the swarm drops to 0 hit points or dies, there bodies are destroyed but there essence travels back to there domain in the Nine Hells, and they are unable to take physical form for a time.

Innate Spellcasting (30/Day). The swarm can innately cast divine word (spell save DC 26). there spellcasting ability is Charisma.

Legendary Resistance (50/Day). If the swarm fails a saving throw, they can choose to succeed instead.

Limited Magic Immunity. Unless they wishes to be affected, The swarm is immune to spells of 6th level or lower. they have advantage on saving throws against all other spells and magical effects.

Magic Weapons. The swarm's weapon attacks are magical.

Multiple Heads. The swarm can take one reaction per turn, rather than only one per round. they also have advantage on saving throws against being knocked unconscious. If they fail a saving throw against an effect that would stun a creature, one of there unspent legendary actions is spent.

Regeneration.  The swarm regains 300 hit points at the start of there turn.

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a gargantuan tiamat. The swarm gain temporary hit points. 

Multiattack. The swarm can use there Frightful Presence. they then make thirty attacks: twenty with there claws and ten with there tails.

Claws. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 130 (40d6 + 10) slashing damage.

Tails. Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 170 (40d8 + 10) piercing damage.

Frightful Presence. Each creature of The swarm’s choice that is within 720 feet of The swarm and aware of them must succeed on a DC 52 Wisdom saving throw, If the saving throw fails by 15 or more, the creature is instantly die of fear, otherwise it becomes 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 swarm's Frightful Presence for the next 24 hours.

The swarm can take 50 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 swarm regains spent legendary actions at the start of there turn. The swarm’s legendary action options are associated with there five dragon heads (a bite and a breath weapon for each). Once The swarm chooses a legendary action option for one of there heads, they can’t choose another one associated with that head until the start of there next turn.

Bite(Costs 3 Actions).. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 210 (40d10 + 10) slashing damage plus 14 (40d6) acid damage (black dragon head), lightning damage (blue dragon head), poison damage (green dragon head), fire damage (red dragon head), or cold damage (white dragon head).

Black Dragon Head: Acid Breath (Costs 20 Actions). The swarm breathes acid in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 54 Dexterity saving throw, taking 670 (150d8) acid damage on a failed save, or half as much damage on a successful one.

Blue Dragon Head: Lightning Breath (Costs 20 Actions). The swarm breathes lightning in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 54 Dexterity saving throw, taking 880 (160d10) lightning damage on a failed save, or half as much damage on a successful one.

Green Dragon Head: Poison Breath (Costs 20 Actions). The swarm breathes poisonous gas in a 90-foot cone. Each creature in that area must make a DC 54 Constitution saving throw, taking 770 (220d6) poison damage on a failed save, or half as much damage on a successful one.

Red Dragon Head: Fire Breath (Costs 20 Actions). Tiamat breathes fire in a 90-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 91 (26d6) fire damage on a failed save, or half as much damage on a successful one.

White Dragon Head: Cold Breath (Costs 20 Actions). The swarm breathes an icy blast in a 90-foot cone. Each creature in that area must make a DC 54 Dexterity saving throw, taking 720 (160d8) cold damage on a failed save, or half as much damage on a successful one.