It's standard legendary action is to attack with one of it's hands at random.

Ah. Lemme fix something real quick. The Meowsma has a +12 to hit on all of its melee attacks, due to having a Strength mod of +7 and a proficiency bonus of 5. All CR 15 monsters have a proficiency bonus of 5. So again, all of its melee attacks have a +12 to hit instead of +10.

Upon hitting an attack with the Elf arm, The Meowsma can cast one cantrip from the Wizard or Ranger spell lists.

The elf arm also carries a scimitar.

*Yay, we got the arms done!*

The meowsma has proficiency in the Stealth and Perception skills.

Legendary action that costs two points is called Hallucinogenic Horror, a mental attack deals psychic damage only to all creatures under the effect of its miasma (so creatures paralyzed, petrified, or frightened by it).

Innate spellcasting. The meowsama uses its intelligence stat as it’s spellcasting modifier 

It should be able to bite (use the bite of the purple thing that's a gargantuan beast (monstrosity but called a smth beast)), and be able to turn into most DND cats

Since Tara added two things and that’s technically not allowed, I will use this post to suggest giving it a shapechange feature that allows it to turn into a beast or monstrosity of any CR up to 15, but it must be a cat.

Quote from tarasque2897 >>

It should be able to bite (use the bite of the purple thing that's a gargantuan beast (monstrosity but called a smth beast)), and be able to turn into most DND cats

 [Okay, so the bite and becoming cats are definitely two different aspects, but although the bite was suggested first, Imma keep the ability to disguise itself as a regular cat as that works well with it's Mimicry. Also, did you mean the Purple Worm? If so, that probably wouldn't work well since the size and damage doesn't really match up (it's also got enough attacks as is lmao) ]

The Meowsma: Huge Abberation (Alignment Undecided)

Description: The Meowsma has eight arms from assorted races: one human arm, one dwarf arm, one elf arm, one orc arm, one firbolg arm, one tiefling arm, one bugbear arm, and one goblin arm. It has the nature of an Eldritch Horror, though it natural sounds like a distorted and otherworldly cat that matches it’s head, which is one of a mangy cat that looks like it’s been on the street for years, and the rest of its feline body follows suit, as it is equally emaciated, long and spindly from what looks like malnourishment. The Meowsma is a formidable foe, using it’s ability to phase in and out of existence to sneak up on prey, or lure them in with the cries of previously devoured adventurers using its Mimicry trait. Once within combat range, it uses it’s highly effective miasma to either immobilise foes, or wreak mental havoc on them with illusory threats.

Stats:

STR: 24  DEX: 15  CON: — WIS: 15 INT: 25  CHA: —
Saving Throw Proficiencies: WIS
Skill Proficiencies: Stealth, Perception

AC: 18   CR: 15   PB: +5

40ft. Walking Speed, 80ft. Climbing Speed

Traits
Mimicry: The Meowsma can mimic any sounds it has heard, including voices. A creature that hears the sounds can tell the are imitations with a successful DC 14 Wisdom (Insight) check.

Spider Climb. The Meowsma can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Special Equipment. The Meowsma carries a Longsword Of Wounding in its human hand.

Corruption. Once the Meowsma has taken (undecided) points of damage, it begins to leak corruption. Any creature that starts their within 10 feet of the Meowsma must make a DC (undecided) saving throw or gain one level of Exhaustion. If a creature dies whilst it has any levels of this exhaustion, it is resurrected as a Shadow on the Meowsma’s turn.

Actions
Multiattack. The Meowsma makes two random attacks with two of its eight arms. Roll 2d8, and reroll 1d8 if the numbers match, to determine which arms the Meowsma attacks with. The arms correlate in order of how they are presented below with the numbers on the d8.

Human Arm. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: 1d8 + 7 slashing damage. At the start of each of the wounded creature's turns, it takes 1d4 necrotic damage for each time you've wounded it, and it can then make a DC 15 Constitution saving throw, ending the effect of all such wounds on itself on a success. (See Longsword of Wounding for full details)

Orc Arm. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: (undecided) damage, and any creature hit that is Large or smaller has to succeed on a DC (undecided) Strength Saving throw or be pushed back 10 feet and fall prone.

Tiefling Arm. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: (undecided) damage, and the creature must succeed on a DC (undecided) Constitution Saving throw, or take (undecided) fire damage at the start of each of the Meowsma’s turns. At the start of the creatures turn, they can repeat this saving throw. On a success, the effect ends. 

Goblin Arm.  Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: (undecided) damage, and the Meowsma steals one item off of the creature. This can be armour, weapons, objects or gold. The player must succeed on a Wisdom (Perception) check contested by the Meowsma’s Dexterity (Sleight Of Hand) check. On a success, the Meowsma does not succeed in stealing the item, and on a failure, the item is stolen without the creature realising.

Firbolg Arm. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: (undecided) damage, and the creature must succeed on a DC 15 Strength saving throw or suffer the effects of the Earthbind spell. 

Bugbear Arm. Melee Weapon Attack: +12 to hit, reach 10 ft., one creature. Hit: (undecided) damage, and the creature must succeed on a DC (undecided) Wisdom Saving throw, or the Meowsma is invisible to the creature until the start of it's next turn.

Elf Arm With Scimitar. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: 1d6 + (undecided because of Finesse) damage. The Meowsma can then also immediately cast one cantrip from either the Wizard or Ranger Spell List.

Dwarf Arm. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: (undecided) damage, and the creature becomes one size smaller and has its speed reduced by 10 feet (to a minimum of Tiny and 0 feet movement speed.)

Miasma (Recharge 5-6). The Meowsma exhales a miasma of red smoke in a 30-foot cone. Roll a D4. On an odd number, each creature in that area must succeed on a DC 13 Constitution saving throw. On a successful save, the target is paralysed until the end of it’s next turn. On a failed save, a target begins to turn to stone and is paralysed, and must repeat the saving throw at the end of its next turn. On a success, the effect ends on the target. On a failure, the target is petrified until freed by the Greater Restoration spell or other magic, however they are still conscious and aware of their surroundings and therefore can feel pain, though they still do not take damage like regular Petrification. 

On an even number, each creature in that area takes (undecided) psychic damage and is frightened of the Meowsma for a minute, as they suffer horrifying hallucinations.

Bonus Actions 
Ethereal Jaunt. The Meowsma shifts from the Material Plane to the Ethereal Plane, or visa versa. 

Legendary Actions
The Meowsma can take (undecided) 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 Meowsma regains spent legendary actions at the start of its turn.

Arm. The Meowsma makes one random arm attack (roll a d8 to determine which arm)

Hallucinogenic Horror (Costs 2 Actions). All creatures under either effect of the Meowsma's Miasma takes (undecided) psychic damage.

[Imma leave the cat transformation thing and bite for now as it's still under discussion, hopefully I didn't miss anything else]

Let’s flesh out these attributes. Charisma 14

The orc arm deals 2d6 + 7 bludgeoning damage on a hit.

16 Constitution.

Immune to Poison damage

It also has proficiency in Strength and Intelligence saves.

Immunity to psychic damage.

*boink

I'd add an aspect but that would be two in a row*

The Meowsma can take 3 legendary actions.

Immune to the frightened condition. 

Immunity to being charmed