Ancient True Hydra

Mythic Resistance (5/5 hours). If the Ancient True Hydra fails on a saving throw of any kind, it can choose to succeed.

Astral Ignorance. The Ancient True Hydra has no use for materials, due to being in the Astral Plane. As such, the beast has no need to use material components for any spell, as well as verbal components, regardless of the spell consuming a component or not.

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

Whenever the hydra takes 150 or more damage in a single turn, one of its heads dies. If all its heads die and it doesn't regenerate, the hydra dies.

At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it takes necrotic damage in the turn that kills the head. The hydra regains 50 hit points for each head regrown in this way. Also, for whatever "elemental" head is killed, two more of that same "elemental" type is regrown.

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.

Wings of the Astral One. The Ancient True Hydra has the ability to fly due to three pairs of massive astral wings. It has a flight speed of 100 when using its wings.

Hold Breath. The hydra can hold its breath for an hour for each head it has.

Piercing Strike. The Ancient True Hydra's attacks, magical and nonmagical, pierce through resistances and immunities that would otherwise be effective.

Ability Expansion. The range and duration (area of effect as well, if applicable) of all abilities that the Ancient True Hydra possesses is expanded by triple the normal amount. The ability, if it's a spell taken from the main spell list, is also able to be casted once per head. i.e, if all thirteen heads are still alive, ray of frost is cast thirteen times, once from each head. It can also multitarget because of this.

Multitasking. The Hydra has no need for concentration due to the fact that it has thirteen heads.

Spellcasting. The Ancient True Hydra is a 20th-level spellcaster. Its spellcasting ability is [Wisdom] (spell save DC 20, +15 to hit with spell attacks). The Ancient True Hydra has a main spell list cast by any head, then has signature spells utilizing each head (each head has its own spells, but not spell slots, it takes from the main spell list slots, which is why there's so many slots on the main spell list). Each turn, the Hydra's spell slots are reset

Main spell list:

Cantrips (at will): [eldritch blast], [thaumaturgy], [true strike], [vicious mockery], [ray of frost], [poison spray], [fire bolt], [resistance]
1st level (15 slots): [hellish rebuke], [create or destroy water], [magic missile], [mage armor], [sleep], [shield], [grease], [absorb elements]

2nd level (15 slots): [enthrall], [gust of wind], [hold person], [ray of enfeeblement], [misty step], [shatter], [silence], [web], [spike growth] 

3rd level (15 slots): [bestow curse], [fireball], [magic circle], [meld into stone], [slow], [glyph of warding], [haste], [counterspell]

4th level (15 slots): [black tentacles], [compulsion], [confusion], [banishment], [greater invisibility], [guardian of faith], [stoneskin] 

5th level (15 slots): [cloudkill], [dispel evil and good], [hallow], [telekinesis], [wall of force] 

6th level (15 slots): [flesh to stone], [eyebite], [disintegrate], [forbiddance], [wind walk], [investiture of wind]

7th level (15 slots): [forcecage], [prismatic spray], [reverse gravity], [regenerate], [teleport], [symbol]

8th level (15 slots): [power word stun], [control weather], [maze], [mind blank],  

9th level (15 slots): [wish], [prismatic wall], [power word kill], [imprisonment], [shapechange], [time stop]

Flame Head Signature Spells:

2nd level: [heat metal], [flaming sphere], [scorching ray]

4th level: [fire shield], [wall of fire]

5th level: [flame strike], [immolation]

6th level: [investiture of flame]

7th level: [delayed blast fireball], [fire storm]

8th level: [incendiary cloud]

9th level: [meteor swarm]

Lightning Head Signature Spells:

3rd level: [lightning bolt], [call lightning]

4th level: [storm sphere]

6th level: [chain lightning]

9th level: [storm of vengeance]

Earth Head Signature Spells:

4th level: [stone shape]

5th level: [wall of stone]

6th level: [wall of thorns], [investiture of stone]

Water Head Signature Spells:

3rd level: [water walk], [wall of water]

4th level: [control water], [watery sphere]

Shadow Head Signature Spells: 

2nd level: [darkness], [blindness/deafness], [silence]

3rd level: [fear]

4th level: [greater invisibility], [phantasmal killer], [black tentacles]

Acid Head Signature Spells:

2nd level: [acid arrow]

4th level: [vitriolic sphere]

9th level: [storm of vengeance]

Death Head Signature Spells:

1st level: [inflict wounds]

3rd level: [vampiric touch], [spirit guardians]

4th level: [blight]

6th level: [circle of death], [harm]

7th level: [finger of death]

Mind Head Signature Spells:

1st level: [bane], [command], [hideous laughter]

2nd level: [suggestion]

5th level: [dominate person], [geas]

6th level: [mass suggestion]

8th level: [antipathy/sympathy], [feeblemind], [dominate monster]

Ice Head Signature Spells:

4th level: [fire shield], [ice storm]

5th level: [cone of cold]

6th level: [wall of ice], [freezing sphere], [investiture of ice]

9th level: [storm of vengeance]

Light Head Signature Spells:

1st level: [guiding bolt]

2nd level: [moonbeam]

3rd level: [spirit guardians]

6th level: [sunbeam]

8th level: [sunburst]

Antimagic Head Signature Spells: 

3rd level: [dispel magic], [counterspell]

5th level: [antilife shell], [dispel evil and good], [planar binding]

6th level: [globe of invulnerability], [guards and wards]

8th level: [antimagic field], [holy aura]

9th level: [imprisonment]

Illusion Head Signature Spells: 

1st level: [silent image]

2nd level: [blur], [enlarge/reduce], [mirror image], [silence]

3rd level: [hypnotic pattern], [major image]

4th level: [hallucinatory terrain]

5th level: [dream], [mislead]

6th level: [programmed illusion]

7th level: [mirage arcane], [project image], [simulacrum]

9th level: [weird]

Health Head Signature Spells: 

3rd level: [vampiric touch]

6th level: [heal]

7th level: [regenerate]

9th level: [wish], [true polymorph]

Elemental Strike. One of the Hydra's elemental heads can make a spell attack, or a breath attack dealing damage of its type: Flame - Fire, Lightning - Thunder, Earth - Force, Water - Cold, Shadows - Necrotic, Acid - Acid, Death - Necrotic, Mind - Psychic, Ice - Cold, Light - Radiant.

Multiattack. The Hydra makes as many bite attacks as it has heads; The Hydra can substitute a quarter of its attacks(minimum of 1) to make an elemental head attack.

Bite. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 32 (4d10 + 9) piercing damage plus an additional 7 (2d6) poison damage..

Poison Breath. The Ancient True Hydra heads exhale a poisonous sludge in a 90-­-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, taking 77 (22d6) poison damage on a failed save, or half as much damage on a successful one.

Self Sever. The Ancient True Hydra bites off one of its own heads, allowing for regeneration of the head on the next turn.

The Hydra can take 5 mythic actions, choosing from the options below. Only one mythic action option can be used at a time and only at the end of another creature's turn. The Hydra regains spent mythic actions at the start of its turn.

Strike of the Damned (Costs 1 Action). The Ancient True Hydra casts magic missile from all of its heads, ignoring spell slots. It can either focus all of its strikes on one target or attack multiple targets at once.

Shield of the Astral King (Costs 2 Actions). The Ancient True Hydra casts prismatic wall from all of it's heads, ignoring spell slots. It can create an entire spherical shield using this ability if it has enough heads.

Cruel Clouds (Costs 3 Actions). The Ancient True Hydra casts cloudkill from half of its heads (minimum of 1), ignoring spell slots. The range and area of effect stacks.

Astral Magic Eradication (Costs 4 Actions). The Ancient True Hydra calls upon its astral powers to strip all creatures within a 2 mile radius of all magical abilities. This turns magical weapons into nonmagical ones, forbids all magic casters from using their abilities for 3 turns each, and shuts down any magic buffs or traps set. This ability cannot be undone by a wish spell or any magic under it.

Hollow Purple (Costs 5 Actions). The Ancient True Hydra casts disintegrate from all of its heads, ignoring spell slots. It can choose to focus all attacks on one target or attack multiple targets at once. (CREATORS NOTE: this will insta kill whatever it is attacking, so use at your own risk. Keep in mind that this creature is a 20th level spell caster, so disintegrate deals massive damage at this level, not to mention that it's being multiplied by however many heads this thing has.)