Core Hydra

Core Meltdown (Recharges after one week). When the hydra is reduced to 0 hit points, it doesn't die or fall unconscious. Instead, its cores begin to release an immense amount of magical energies, vaporizing the Hydra's physical body and creating a construct of pure energy. The hydra has six cores, one for each of its original heads  and a central core that lies within its chest. When active, each elemental core creates a head of that pure element and is immune to its respective element (Fire, Cold, Lightning, Necrotic), while the 5th head core is simply immune to all magical and nonmagical ranged attacks due to the high pressure winds it creates. Each elemental core has an AC of 16 and 50 hit points. When a core is destroyed, the hydra loses an elemental head. The hydra's central core is immune to all damage until the elemental cores are destroyed. Otherwise, it has an AC of 20 and 175 hit points. When all they Hydra's cores are destroyed it dies. If any of the core's must make an ability saving throw, treat its ability scores as 10 (+0).

Over the time period of one week, the cores will slowly reconstruct the body of the hydra and heal it completely, restoring any destroyed cores in the process.

Legendary Resistance (2/Day). Whenever the Core hydra fails a saving throw, it can choose to succeed instead.

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

Multiple Heads. The hydra has five heads. While it has more than one head, the hydra is immune to being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.  Whenever it takes 40 or more damage in a single turn, one of its heads dies. If all of its heads die - it immediately drops to 0 hit points and triggers its mythic trait "Core Meltdown". At the end of its turn, it grows two heads for each of its heads that died since its last turn. The hydra regains 20 hit points for each head regrown in this way. 

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

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

Multiattack. The Core hydra can make as many bite attacks as it has heads.

Bite. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 28 (4d8 + 10) piercing damage. 

If the hydra's "Core Meltdown" trait is active, this attack deals an additional 5 (2d4) damage of the appropriate element type with the exception of the gale head which also knocks creatures prone on a successful hit.

Stomp. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) bludgeoning damage.

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Breath Weapons (Recharge 5–6). The hydra uses one of the following breath weapons.

Fire breath. The hydra exhales fire in a 60-foot cone. Each creature in that area must make a DC 20 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.

Frozen Impact. The hydra targets a space within 80ft of itself lets lobs a sphere of freezing energy from its maw. On impact, the sphere explodes in a 20 foot radius. Each creature in that area must make a DC 20 Dexterity saving throw, taking 63 (18d6) cold damage on a failed save, or half as much damage on a successful one. For 1d4+1 rounds after, the ground is frozen over and considered difficult terrain. The ice melts early if a source of extreme heat, such as the hydra's fire breath, comes in contact with it.

Crackling Lightning. The hydra unleashes a giant bolt of electricity from its maw in a 15 ft. wide by 200 ft. long straight line. Each creature in that area must make a DC 20 Dexterity saving throw, taking 63 (18d6) lightning damage on a failed save, or half as much damage on a successful one.

Ennervating Surge. The hydra exhales a dark purple cloud of malignant energies directly at its feet which surges out into a 20 ft radius sphere around it. This cloud spreads around corners and covers. Each creature in that area must make a DC 20 Dexterity saving throw, taking 63 (18d6) necrotic damage on a failed save, or half as much damage on a successful one. This gas lingers for 1d4+1 rounds after and functions as magical darkness for the duration, but can be dissipated by a wind of moderate or greater speed (at least 10 miles per hour).

 Gale Force. The hydra exhales high pressure winds in an 80-foot cone. Each creature in that area must make a DC 20 Strength saving throw, on a failed save they are flung 60 feet into the air by the tornado like forces, on a successful save they are pushed backwards 10 feet.

Shifting Scales. When hit by an attack, the core hydra can use its reaction to immediately gain resistance to that damage type until the end of its next turn. Alternatively, it can choose to become invisible until the end of its next turn.

The Core hydra has 2 legendary actions, choosing from the options below. Only one legendary option can be used at a time and only at the end of another creature's turn. The hydra regains any spent legendary actions at the start of its turns and gains an additional legendary action for every head it has above 3.

Detect. The hydra makes a Wisdom (Perception) check.

Bite. The hydra makes a bite attack against one creature within range.

If the Core Hydra's mythic trait is active it can use the following options below as legendary actions for 1 hour after using "Core Meltdown".

Elemental Storm (Costs All Actions) (Recharges after 10 rounds). The hydra unleashes a torrent of elemental energies in a 60-foot cone. Each creature in that area must make a DC 20 Dexterity saving throw, taking 24 (6d6) damage from each living head on a failed save, or half as much damage on a successful one.