Violet, The Death Slime (Magical Girl)

Acidic Body. Violet's body is fully made out of acid which can get melt and digest anything. Upon touching a creature takes 1d6 acid damage. Magical Items are unaffected and magical potions that get consumed take effect.

Enhanced Regeneration. Violet heals for 1d4 for every 5 cubic feet of material that she consumes. If the material is organic she heals for 1d10.

Size Regulation. Violet can vary her size from normal 6 feet in height up to 100 feet in height and 50 feet in diameter at the base.

Spellcasting. Violet is a spellcaster, thanks to being a Magical Girl. Her spellcasting ability is charisma (spell save DC 18, +10 to hit with spell attacks). Violet has following spells prepared:
Cantrips (at will): Eldritch Blast (4 Rays), Acid Splash
1st level (5 spell slots): Cause Fear, Bane, Hex, Arms of Hadar
2nd level (4 spell slots): Ray of Enfeeblement, Hold Person
3rd level (3 spell slots): Fear, Fly, Gaseous Form
4th level (2 spell slots): Blight, Hallucinatory Terrain
5th level (1 spell slot): Negative Energy Flood, Danse Macabre
6th level (1 spell slot): Eyebite, True Seeing
7th level (1 spell slot): Power Word Pain, Crown of Stars

Multi-attack. Violet attacks twice per round. Only Slam and Swipe can be used this way.

Slam. Melee Weapon Attack: +13 to hit, reach 5-40(depending on size) ft., 1 target. Hit. 3d10 + 13 bludgeoning damage.

Swipe. Melee Weapon Attack: DEX saving throw DC = 18, reach 5-20(depending on size) foot radius, Failed 3d10 + 13 bludgeoning damage. Half as much on a success.

Burst. Instead of attacking twice, Violet can instead launch a series of smaller slimy tentacles towards all enemies within 5-30(depending on size) feet of her. DEX saving throw DC = 18. On a failed save an enemy takes 4d10 + 13 bludgeoning damage and just as much acid damage. On a successful save they only take half of the bludgeoning damage and no acid damage.

Enter the description for how legendary actions work for your monster here.

Slimy Explosion (Can only be used when she is at least 50 feet tall). At the end of her turn Violet starts to violently shake, charging up an attack. If she doesn't take 100 or more points of damage by the start of her next turn, she will send giant slimy tentacles flying towards every enemy within 120 feet of her. Enemies must make a DEX saving throw DC = 18. On a failed check they take 5d10 bludgeoning damage, are grappled and are pulled inside of her. When they start their turn inside of her, they have halved speed and take 5d10 Acid damage, healing Violet for 20% of the damage dealt. On a successful check they only take 5d10 bludgeoning damage.