Greeed Overlord

Name: Greeed Overlord
Size: Gargantuan
Type: Monstrosity
Alignment: Lawful Evil
Stats: STR 30 (+10) DEX 15 (+2) CON 10 (+0) INT 20 (+5) WIS 25 (+7) CHA 30 (+10)
Challenge Rating: 30 (+9)
Armor Class: 19
Hit Points: 525 (50d20 + 0)
Speed: 10 ft. walking, 80 ft. flying, 40 ft. swimming
Resistances/Immunities/Vulnerabilities: Resistance to Slashing, Piercing, and Bludgeoning. Vulnerable to Thunder and Lightning damage when in flight.
Saving Throws: CON +9 INT +14 WIS +16 CHA +19
Skill Bonuses: Insight +16, Persuasion +28
Senses: Darkvision 120 ft. Truesight 10 ft. (Passive Perception: 17)
Languages: Any (Telepathy)

Actions:
Multiattack: The Overlord can make six attacks, two with its Punches, two with its claws, one with its Talon, and one with its Tentacles.
Punch, +19 to hit, reach 20 feet, one target. Hit: 27 Bludgeoning Damage (5d6 + 10), when Grey Core Medals are gone, halve the damage.
Claw, +19 to hit, reach 10 feet, one target. Hit: 28 Slashing Damage (4d8 + 10), when Yellow Core Medals are gone, halve the damage.
Talon, +19 to hit, reach 5 feet, one target. Hit: 26 Piercing Damage (3d10 + 10), if target is hit, it must make a DC 20 Strength Saving Throw or be grappled. When Red Core Medals are gone, halve the damage and DC.
Tentacle, +11 to hit, Reach 30 feet, four targets. Hit: 15 Lightning Damage (2d12 + 2), if target is hit, it must make a DC 20 Strength Saving Throw or be grappled. When Blue Core Medals are gone, halve the damage and DC.
Flight: The Greeed Overlord can stay in flight at the end of its turn.

Spells: The Greeed Overlord can, at the expense of an appropriate Core Medal, cast a spell. If it does this, however, it opts out of multiattack. Save DC is 22.
Mirror Image (Cost: 1 Green Core Medal)
Immolation (Cost: 1 Red Core Medal)
Frightful Presence (Cost: 1 Yellow Core Medal)
Reverse Gravity (Cost: 1 Grey Core Medal)
Watery Sphere (Cost: 1 Blue Core Medal)
Meteor Swarm (Cost: 1 Red and 1 Grey Core Medal)
Destructive Wave (Cost: 1 Green and 1 Yellow Core Medal)

Other Stuff:
Core Medals: This beast is made out of fifty Core Medals, each of which is what composes its very being. When hit by a magic weapon, make a DC 20 Constitution Saving Throw. If the Overlord succeeds, it simply gets damaged, but if it fails, then a Core Medal is taken out of the monster and it takes 10 Psychic damage (d20). If it's a critical hit, regardless of if the weapon is magical or not, then the Core Medal is removed without any save. If all the Core Medals are removed this way, then the monster is instead killed. It can also lose Core Medals through casting spells, complete with taking damage.
Lightning Rod: Whenever it suffers Thunder or Lightning Damage, it gets a reaction to create a sixty foot radius of lightning bursts. Those affected in the radius must make a DC 22 Reflex Saving Throw. Those who fail take 55 Lightning Damage (10d10). It can't do this when the Green Cores are gone.

Legendary Actions: The Greeed Overlord 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 Greeed Overlord regains spent legendary actions at the start of its turn.
Zap: The Greeed Overlord can zap whatever it's holding with its tentacles. All grappled by the tentacles take 18 Lightning Damage (2d12 + 5)
Burn: The Greed Overlord can burn whatever it's holding with its talons. The person grappled takes 21 Fire Damage (3d10 + 5)
Temptation (2 Actions): The Greeed Overlord can choose one target within 120 feet. It will know their greatest desires and will make an offer. The affected must make a 27 DC Wisdom Saving Throw. If they succeed, they ignore the temptation and the Overlord can't pick them again for the rest of the encounter. If they fail though, they will, for one minute, engage in a mental representation of their desires coming to life. Snapping them out of it through an attack will give them 17 Psychic Damage (D20 + 7).