Cannibraingia

Hold Breath. While out of water, the kaiju can hold its breath for 1 day.

Underwater Camouflage. The kaiju has advantage on Dexterity (Stealth) checks made while underwater.

Water Breathing. The kaiju can breathe only underwater.

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 poison damage before the the start of it’s turn, or its hit point total drops to 0.

Uncontrollable Rampage. Every few decades, it becomes unaligned and goes on a rampage. While it is on a rampage, it has resistance to all damage except force cold and thunder. In addition, while rampaging, it cannot cast spells. This effect lasts for 1d6 weeks. The effect can be ended early if calm emotions is cast on the kaiju (the kaiju is considered a humanoid for the purpose of this spell).

Innate Spellcasting (Psionics)

The kaiju's spellcasting ability is Intelligence (spell save DC 22). It can innately cast the following spells, requiring no components:

At will: mage hand (the hand is invisible), telekinesis, arcane eye, blur, clairvoyance, detect magic, detect thoughts, levitate, shield

3/day each: arcane hand (the hand is invisible), hold monster, modify memory, project image, wall of force

1/day each: astral projection, scrying, mind blank (self only), force cage

Multiattack. The kaiju can use its frightful presence. It then makes five attacks: one with its bite and four with its tentacles. It can replace one attack with acid ink.

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.

Tentacle. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 16 (2d8 + 7) bludgeoning damage. If the target is a creature, it is grappled (escape DC 25). Until this grapple ends, the target is restrained. The kaiju has eight tentacles, each of which can grapple a creature, until the grapple ends, the kaiju can't use this tentacle on another target.

Bite. Melee Weapon Attack: +16 to hit, reach 10 ft., one creature. Hit: 33 (4d12+7)piercing damage. The target is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the kaiju can't bite another target.

Ink Cloud (Recharges after a Short or Long Rest). A 100-foot-radius cloud of ink extends all around the kaiju if it is underwater. The area is heavily obscured for 1 minute, although a significant current can disperse the ink. After releasing the ink, the kaiju can use the Dash action as a bonus action.

Acid Ink. Ranged Weapon Attack: +10 to hit, range 200 ft., one creature. Hit: 20 (4d6+10) acid damage.

Psionic Beam (Recharge 5-6). The kaiju emanates a line of energy from its head that is 10 feet wide and 1 mile long. Each creature in that area must make a DC 21 Inteligence saving throw, taking 63 (18d6) psychic 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 (except bite).

Move. The kaiju moves up to half its speed.

Chomp. (Costs 2 Actions). The kaiju makes one bite attack.