Innate Spellcasting. The Accursed Grimalkin's innate spellcasting ability is Wisdom (spell save DC 24, +15 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: Acid Splash, Eldritch Blast
3/day each: Feeblemind, Slow, Dominate Monster, Greater Invisibility (range self), Psychic Crush (8th level), Mental Prison, Mirage Arcane, Arms of Hadar (8th level), Hunger of Hadar, Earthquake, Tsunami, Whirlwind, Incendiary Cloud, Antimagic Field, Maze, Maddening Darkness, Firestorm (8th level)
1/day each: Power Word Kill, Meteor Swarm, Storm of Vengeance, Psychic Scream, Weird

I Am One With Nature. The Accursed Grimalkin can phase through solid matter.

Limited Magic Immunity. The Accursed Grimalkin is immune to spells of 6th level and lower unless it chooses to be affected and has advantage on saving throws against magical effects.

Heaven Is Void Of Light. The Accursed Grimalkin can tear the seams between the different realms and dimensions, going through as it pleases. This takes up two actions, opening up a portal and then going through it.

Immutable. The Accursed Grimalkin is immune to any spell or effect that would alter its form.

Open Your Eyes. The Accursed Grimalkin's eyes glow, and anyone who stares at them for too long will start seeing horrors that they cannot comprehend. His eyes can faintly see other dimensional planes and anyone inhabiting them.

Embodiment of Unreality. Stopping time won't restrict them; it will still be able to move.

I Am Eternal. If the Accursed Grimalkin is reduced to 0 hit points it can make a constitution saving throw DC 5+ the damage taken, if a success it goes to 1 hit point. If it’s killed, only it’s physical shell is destroyed. It will reform in 1d10x4 years in Mytheria.

Maddening Visage. A creature that starts its turn and is able to see the Accursed Grimalkin must succeed on a DC 24 Wisdom saving throw or become frightened until the beginning of their next turn. If a creature's saving throw is successful, the creature is immune to this ability for the next 24 hours.

I Can Always See You. The Accursed Grimalkin has advantage on perception checks and has perfect vision from any range.

Disgustingly Resilient The Accursed Grimalkin regains 35 hit points at the beginning of each of its turns if it didn’t take force or radiant damage on the previous turn and if the Accursed Grimalkin gets hit by an attack that does 25 damage or less – it instead does no damage.

I Am Annihilation. The Accursed Grimalkin deals double damage to objects and structures and when scoring critical hits it rolls 3x as many damage dice.

Eldritch Fur. Any time the Accursed Grimalkin is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, it is unaffected. On a 6, it is unaffected, and the effect is reflected back at the caster as though it originated from the Accursed Grimalkin, turning the caster into the target. 

Titan of Eldritch Might. The Accursed Grimalkin has advantage on all strength and constitution checks.

Otherworldly Howl (Recharge 5-6). The Accursed Grimalkin emits a soul-splitting howl out loud and telepathically. Each creature in a 240 foot radius around Garfield must make a DC 24 Constitution saving throw, taking  (8d10) thunder damage on a failed save and being deafened until the end of its next turn, or half as much on a successful save. Each creature must also make a DC 24 Intelligence saving throw or take (8d10) psychic damage on a failed save, or half as much on a successful save. A deafened creature does not take thunder damage from this attack.

Acid Hairball (Recharge 5-6). Ranged Attack: +15 to hit, range 240 ft., one target. Hit:  (5d12 +8) acid damage, (5d12+8) bludgeoning damage. 

Life Was Made For Me To Devour (3/Day). The Accursed Grimalkin lays a curse on a single target.

• Turn 1: After taking an action, the affected creature must make a Charisma saving throw (DC 22). On a failed save, the creature smells of rotten cheese and meat, and they begin to feel queasy (Disadvantage on Dexterity checks and spell concentration rolls). On a successful roll, the effect ends.
• Turn 2: After taking an action, the affected creature must make a Charisma saving throw (DC 23). On a failed save, the creature's skin turns doughy like pasta and they shrink to size tiny (if they weren't already that size). The creature now has disadvantage on Strength checks and loses 3d12 hit points. On a successful roll, the effect ends.
• Turn 3: After taking an action, the affected creature must make a Charisma saving throw (DC 24). On a failed save, all the creature's damage rolls are reduced to 2d4. Their body is now halfway polymorphed into a a writhing mass of meat, and they are knocked prone. On a successful roll, the effect ends.
• Turn 4: After taking an action, the creature must make a Charisma saving throw (DC 25). On a failed save, the creature’s HP total drops to 0, and the creature turns into food, which the Accursed Grimalkin promptly consumes. On a successful roll, the effect ends.

I Am So Hungry...(Mythic Action; 1 per Short Rest). When the Accursed Grimalkin drops to 0 hit points, it can expend it's Mythic Action to regain all of its innate spellcasting as well as 600 hit points. In addition to this, for the duration the Accursed Grimalkin is resistant to all damage

Multiattack. Garfield makes four attacks: two with his claws, and one with his bite and 1 with his tentacles.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft, one target. Hit: (6d12 + 10) piercing damage

Claws. Melee Weapon Attack: +15 to hit, reach 10 ft., two targets. Hit: (4d10 + 10) slashing damage.

Tentacles. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: (3d10+10) bludgeoning damage. If the target is a creature, it is grappled (escape DC 22). If the target is Huge or smaller, it is also restrained until this grapple ends. While grappling the target, The Accursed Grimalkin has advantage on attack rolls against it. The Accursed Grimalkin can grapple a maximum of four creatures at a time. When The Accursed Grimalkin moves, any Huge or smaller target it is grappling moves with him.

Swallow. The Accursed Grimalkin makes one bite attack against a huge 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 Garfield, and it takes (8d6) acid damage and (8d6) bludgeoning damage at the start of each of Garfield's turns. If The Accursed Grimalkin takes 60 damage or more on a single turn from a creature inside it, the Accursed Grimalkin must succeed on a DC 24 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of it. If the Accursed Grimalkin dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

There’s No Escape. When creature comes within 10 feet of the Accursed Grimalkin, it makes a Claws attack.

I Hate Mondays. When the Accursed Grimalkin's hit point total drops to 5, it can perform Acid Hairball twice in one turn as a multiattack, overcoming the recharge limitation. If its hit point total rises above 5 again, this effect wear off. 

The monster can take 5 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 monster regains spent legendary actions at the start of its turn.

Attack (Costs 1). It makes one tentacle, bite, or claw attack.

Unleashed Anger (Costs 2 actions). It uses its Otherworldly Howl.

Insatiable Hunger (Costs 2). It will move to an unconscious creature and consume its soul and body, healing it for (8d8) hit points. The creature dies and cannot be resurrected except via a Wish spell, until the Accursed Grimalkin chooses to release its soul, or until the Accursed Grimalkin is reduced to 0 hit points.

Hungry Gaze (Costs 3). A creature of the Accursed Grimalkin's choice that is within 240 feet of it must succeed on a DC 23 Intelligence saving throw. On a failed save, the creature is racked with hunger pains and takes (6d10) psychic damage.

If the Accursed Grimalkin's Mythic Trait is active, he can use the options below as legendary actions for the duration.

There Shall Be A New Beginning (Costs 3 Actions). The Accursed Grimalkin releases a wave of destruction, damaging all creatures within 120 feet of it. The target must succeed on a DC 23 Constitution saving throw or take (6d10) necrotic damage and (6d10) force damage. Creatures that fail the saving throw have their very souls crumble, taking (3d10) necrotic damage and the start of their turn. This damage bypasses any damage immunity or resistance and heals the Accursed Grimalkin for the damage dealt. Creatures that die while afflicted by this have their souls reduced to nothingness and can only be restored via the True Resurrection or Wish spell. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. Creatures that succeed on the saving throw take half damage and resist the soul destruction.

I Must Consume (Costs 2 Actions). The Accursed Grimalkin grabs any creature within 10 feet of him and consumes the natural arcane energies from them, they must make a DC 23 Wisdom save. On a failure: if the creature has spellcasting capabilities they must roll a d8 and lose the highest spell slot equal to or less then that number and the Accursed Grimalkin regains one use of one of his 1/day spells, if no spell slots are available treat them as having no spellcasting capabilities, if they have no spellcasting capabilities then they suffer 8d10 psychic damage and the Accused Grimalkin regains one use of one its 3/day spells. If they succeed nothing happens.