Ancient kraken

Amphibious. The kraken can breathe air and water.

Legendary Resistance (4/Day, or 5/Day in Lair). If the kraken fails a saving throw, it can choose to succeed instead.

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

Tidal awakening. When the kraken drops to 0 hit points, it regains 500 hit points and gains use of its Mythic Actions. It also grows natural plate armor, and gains a +3 to its AC

Ancient will. The kraken cannot be teleported, have it's gravity reversed, or be plane shifted unless it is willing.

Immutable form. The kraken cannot be polymorphed/shapeshifted.

Ancient shockwave. At 75%,  50%, and 25% health, it explodes into a shockwave of glory, and all creatures are [condition]Charmed[condition] by it. After 1 turn, they go back to normal, and lose this condition.

Divine shield. At 75%, 50%, and 25% health, it grows a divine shield that requires 15 hits to break. For example, magic missile at level 1 would do 3 damage to the shield.

Aura of Death. At 5, 10, or 15% health (DM's pick), it begins unleashing waves of pure force damage that start in a 20-foot radius around it, and expand by 15 feet per round. The waves deal infinite damage to all creatures within this radius at the end of their turns. This happens only once it has undergone tidal awakening. The type of damage is force, and it bypasses all resistances, immunities, and abilities that would stop the wave of damage from killing it. It can also go through walls, but does not pass through planar portals, so you cannot kill things on the other side unless they come through. This has no limit to how big it can get.

Multiattack. The kraken makes two Tentacle attacks and uses Fling, Lightning Strike, or Swallow.

Tentacle. Melee Attack Roll: +17, reach 30 ft. Hit: 48 (4d12 + 20) Bludgeoning damage. The target has the Grappled condition (escape DC 20) from one of ten tentacles, and it has the Restrained condition until the grapple ends.

Fling. The kraken throws a Large or smaller creature Grappled by it to a space it can see within 60 feet of itself that isn’t in the air. Dexterity Saving Throw: DC 25, the creature thrown and each creature in the destination space. Failure: 28 (8d8) Bludgeoning damage, and the target has the Prone condition. Success: Half damage only.

Lightning Strike. Dexterity Saving Throw: DC 23, one creature the kraken can see within 120 feet. Failure: 66 (6d20) Lightning damage. Success: Half damage.

Swallow. Dexterity Saving Throw: DC 25, one creature Grappled by the kraken (it can have up to four creatures swallowed at a time). Failure: 23 (3d8 + 10) Piercing damage. If the target is Large or smaller, it is swallowed and no longer Grappled. A swallowed creature has the Restrained condition, has Total Cover against attacks and other effects outside the kraken, and takes 24 (7d6) Acid damage at the start of each of its turns.

If the kraken takes 50 damage or more on a single turn from a creature inside it, the kraken must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, each of which falls in a space within 10 feet of the kraken with the Prone condition. If the kraken dies, any swallowed creature no longer has the Restrained condition and can escape from the corpse using 15 feet of movement, exiting Prone.

Abysmal currents (Recharge 6). Strength saving throw DC 20. The kraken calls up currents to suck all who are in the water down into the depths. All creatures that fail the saving throw are dragged 80 feet underwater, and repeat the saving throw at the beginning of their turns, rising 60 feet and escaping the current on a success. Otherwise, they take 1d10 bludgeoning damage per 20 feet sunk, and are suffocating. If pulled more than 500 feet underwater, without being able to breathe water, they then suffocate and die. Otherwise, they die at 700 feet regardless.

Whirlpool (Recharge 4-6). Constitution saving throw DC 15. All ships in a 500-foot radius must have at least half of the whole crew make constitution saving throws, or be sucked into the depths. All sailors are dragged 80 feet underwater, and their ship is destroyed.

Tidal wave. All creatures within a 500 foot long rectangle, 100 feet high, and 250 feet long are put in the path of a tidal wave and take 50 (5d10+10) blugeoning damage, and are knocked prone and unconscious for 30 seconds, and moved to the end of this wave. If they encounter a solid surface, like rocks, this damage is tripled, and they have these conditions for 1 minute. All ships are capsized, and if they are below 75% health on average, they are destroyed.

Spellcasting. The kraken casts one of the following spells, using Intelligence as the spellcasting ability (spell save DC 20):

At will: Shocking grasp (4d8 version), Dimension door, Greater invisibility (Only if underwater, and if ink cloud was used this or last round), and create and control water (1000 gallons), Mind blank

1/day each: Storm of vengeance (1 mile radius), Power word stun, Feeblemind (5 minute duration), Prismatic wall (900 feet long, 300 feet high, 5 feet thick).

Electrical charge. All creatures who deal lightning damage to the kraken have their spell absorbed, and the kraken gains one charge, but takes 1d10 lightning damage. At 3 charges, it can unleash a powerful electrical charge in it's mythic Actions.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the kraken can expend a use to take one of the following actions. The kraken regains all expended uses at the start of each of its turns.

Storm Bolt. The kraken uses Lightning Strike.

Toxic Ink. Constitution Saving Throw: DC 23, each creature in a 15-foot Emanation originating from the kraken while it is underwater. Failure: The target has the Blinded and Poisoned conditions until the end of the kraken’s next turn. The kraken then moves up to its Speed. Failure or Success: The kraken can’t take this action again until the start of its next turn.

Drown. All creatures grapples by the kraken begin suffocating, and if the kraken swims 500 feet underwater with them, the pressure kills them instantly.

Mythic Action Uses: 5 (6 in Lair). Immediately after another creature’s turn, the kraken can expend a use to take one of the following actions. The kraken regains all expended uses at the start of each of its turns.

Electrified wave (costs 2 actions). The kraken uses tidal wave, but adds 50 (5d10+10) lightning damage.

Water jet. The kraken launches all creatures and ships within a 20-foot radius 500 feet into the air. They take 25d6 bludgeoning damage and are knocked prone.

Raise drowned (Costs all actions). The kraken raises up to 10 zombies, ghouls, or skeletons monsters from corpses within a 1000-foot cylinder around it. It can control up to 25 CR worth of undead monsters in this way at a time. It can reanimate another kraken, but only once in it's lifetime.

The following attacks can only be used with electrical charge charges.

Electrify the sea (Costs all actions). All creatures within 1 mile of the kraken in watr take 85 (15d10) lightning damage per turn, for 3 turns. A creature cannot be outright killed by this attack, only begin dying due to how electricity restarts the heart, unless this hits them again while they are dying or unconscious due to this attack.

Wrath of the storm (Costs all actions). The kraken calls forth a gargantuan lightning bolt on a 20 by 20 foot area, so fast that there is no saving throw. All creatures take 150 (25d6 + 12) lightning damage, and are knocked unconscious by the sheer electricity. A creature cannot be outright killed by this attack, only begin dying due to how electricity restarts the heart, unless this hits them again while they are dying or unconscious due to this attack.