Ancient Red Dracohydra

Hold Breath. The kaiju can hold its breath for 1 hour.

Multiple Heads. The kaiju has five heads. While it has more than one head, the kaiju has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever the kaiju takes 25 or more damage in a single turn, one of its heads dies. If all its heads die, the kaiju dies only if it cannot regrow one or more heads and it is below 321 hp, even after regenerating.

At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken at least 25 necrotic damage since its last turn. The kaiju regains 10 hit points for each head regrown in this way.

Reactive Heads. For each head the kaiju has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Wakeful. While the kaiju sleeps, at least one of its heads is awake.

Siege Monster. The kaiju deals double damage to objects and structures.

Magic Resistance. The kaiju has advantage on saving throws against spells and other magical effects.

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

Regeneration. The kaiju heals very quickly, and regains 20 HP at the start of each turn unless it is subject to acid damage before the the start of it’s turn, or its hit point total drops to 0.

Sprouting. The kaiju regrows one head at the end of its turn (and heals 10 hp) if it has zero heads. 

Multiattack. The kaiju uses its frightful presence and makes as many bite attacks as it has heads. It can replace any number of bite attacks with swallow.

Frightful Presence. Each creature of the kaiju’s choice within 1 mile of it and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the kaiju is within line of sight, 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 kaiju’s Frightful Presence for the next 24 hours.

Bite. Melee Weapon Attack: +15:{"diceNotation":"1d20+15","rollType":"to hit","rollAction":"Bite"} to hit, reach 10 ft., one target. Hit: 13 (1d12 + 7) piercing damage and 10 (3d6) fire damage. The target is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the kaiju can't bite another target with this head.

Swallow. 

The kaiju makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the kaiju, and it takes 56 (16d6) acid damage at the start of each of the kaiju's turns. If the kaiju takes 60 damage or more on a single turn from a creature inside it, the kaiju must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the kaiju. If the kaiju dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Breath Weapon (Recharge 5-6). The kaiju exhales a line of energy that is 10 feet wide and 1 mile long. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.

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

Attack. 

The kaiju makes one attack.

 Move. 

The kaiju moves up to half its speed.

 Gulp. (Costs 2 Actions).

The kaiju uses its Swallow (if available).