Star's Radiance. Creatures starting their turn within 60 ft of Yuxxilpoyx must make DC 15 constitution saving throws or take 3d10 fire damage and become blinded until the start of their next turn. Yuxxilpoyx constantly emits 60 ft of bright light and 120 ft of dim light. The light takes on a bloody orange hue.

Immutable Form. This creature cannot be shapeshifted or polymorphed or have its form altered in any way short of a wish spell. 

Stellar Body. This creature is immune to radiant damage, necrotic damage, fire damage, poison damage and effects, disease related effects, and has advantage on saving throws against gravity related effects and spells. Gravity effects that successfully affect Yuxxilpoyx however with have their effects heightened by x2.

Solar Melting. magical items of uncommon rarity or less are absorbed and turned to dust upon contact with Yuxxilpoyx. Non-magical items melt away by being within 30 ft of Yuxxilpoyx.

Permanent Levitation. Yuxxilpoyx is unaffected by effects which affect creatures on the ground.

Spellcasting. The [monster name] is a 24th-level spellcaster. Its spellcasting ability is Constitution (spell save DC 25, +12 to hit with spell attacks). The Hateful Bastard Star has following spells prepared:
Cantrips (at will): Message, Scorching Ray (1 ray), Sunlight, Light, Magnify Gravity (1st level)

5th to 7th level spells are only used when above 50% hp except for Teleport
5th level (5 slots): Chromatic Orb (fire only), Scorching Ray, Gravity Sinkhole, Wall of Fire, Wall of Light, Immolation, Dawn

6th level (5 slots): Sunbeam, Gravity Fissure, Disintegrate, Scorching Ray,  Fireball

7th level (3 slots): Reverse Gravity, Teleport, Crown of Stars, Fire Storm

8th and 9th level spells are only used when reaching 50% hp or less

8th level (2 slots): Incendiary Cloud, Sunburst

9th level (1 slot): Meteor Swarm

below is exclusive to a specific action

10th level (1 slot): Cataract of Fire 

Cataract of Fire. Yuxxilpoyx targets an area within 1 mile of itself. After 1 minute since this action was used, a gigantic pillar of ultra bright white flame slams into the surface of the targeted area, affecting a 300 ft radius 2400 ft high cylinder. All non-magical objects and items caught in the area are instantly turned to ashes. Structures in the area not made of adamantine are incinerated. Barriers of force are simply destroyed when hit. Creatures in the area make a constitution saving throw against Yuxxilpoyx's spell save DC or take 20d8 fire and 20d8 radiant and 10d8 force damage (half on a save). Creatures that die to this spell are reduced to particle specks. This action consumes the 10th level slot and cannot be used again until the equivalent of 1 week of time has passed. Yuxxilpoyx takes the full force damage.

Crush. Yuxxilpoyx focuses on obliterating a creature enveloped by it. It chooses 1 creature considered grappled by it and sends its mass and power into the target. The target is forced to make a DC 25 strength saving throw or take 80 force damage which ignores resistances. A grappled creature cannot use its actions or bonus actions outside of attempting to escape, making the same DC 25 strength saving throw as before on its turn.

Ram. Melee Attack: DC 15 dex save, reach 20 ft line. costs 20 ft of movement. 30 (6d10) fire damage. Half damage on success. 

Envelop. Melee Attack: DC 20 strength save, reach touch. A failed save causes the creature to be considered grappled by Yuxxilpoyx.

Brief Gravitational Shift. Creatures within a 10 ft radius sphere make DC 15 strength saving throws and are forcefully moved up to 20 ft in an unoccupied space of its choice on a failure. This has a range of 60 ft.

Magic Detection Pulse. Yuxxilpoyx becomes aware of all magic that isn't its own within 300 ft of it following the rules for the detect magic spell. This detection state only lasts for 1 round. Yuxxilpoyx cannot use the pulse again until a minute has passed.

Rapid Relocation. Yuxxilpoyx expends a 5th level spell slot to teleport to an unoccupied space within 100 ft of it that it is aware of. 

Solar Rebuke. When Yuxxilpoyx is hit by an attack that deals fire or radiant damage, half of the damage that would be delt is instead redirected towards the attacker who must make a saving throw or attack roll against themself (using their own AC or spell save DC) in order to determine the effects. 

Supernova. Triggers automatically upon Yuxxilpoyx reaching 0 hp and collapsing into itself. Everything within 1000 ft of Yuxxilpoyx takes 60d10 force damage that ignores immunities and resistances. Yuxxilpoyx is considered to be fully destroyed as a result of this.

This creature does not have attack of opportunity 

Yuxxilpoyx takes these legendary actions after the appropriate amount of turns have passed spending the required amount of actions.

Solar Expulsion (Costs 2 Actions). Yuxxilpoyx erupts a 80 ft cone of fiery plasma originating from itself. Creatures within the area must make dexterity saving throws or take 1d10 radiant damage and 2d12 fire damage (half on save). Creatures that fail the save are blinded for 1 minute and are pushed away 30 ft, landing prone. Yuxxilpoyx takes 2d12 untyped damage by using this.

Gather Mass (Costs 1-3 Actions). Yuxxilpoyx pulls material from the world itself in an attempt to regain mass. This material is gathered from up to 300 ft away. Yuxxilpoyx is healed by 5d8+12 hp and can do this up to two more times. If it takes damage from a single source that is greater than the healed amount, Yuxxilpoyx cannot heal the additional times. After doing this three times within 1 week, it cannot do so again until a week's worth of time has passed.

Gather Home Star. Yuxxilpoyx gathers power directly from the Sun, turning a brilliant light blue color. Starting when Yuxxilpoyx has reached below 50% of its total hp, it can use its bonus actions and actions to gain 2d6 stellar charges to use. It no-longer uses any of its legendary actions in this state. It can only have up to a maximum of 12 stellar charges. This state ends when combat ends.

Solar Eruption (Costs 5 Stellar Charges). In either a 200 ft line or a 100 ft cone, creatures in the affected area make dexterity saving throws against Yuxxilpoyx's spell save DC like with Solar Expulsion. Success allows for taking half damage. Failure causes the affected creature/structure/unattended object to take 6d12 fire damage that also counts as force damage for the sake of overcoming resistances and immunities. Affected entities are also pushed 30 ft away from Yuxxilpoyx in the direction of the blast.

Epic Counterspell (Costs 6 Stellar Charges and a reaction). When a creature within 300 ft of Yuxxilpoyx that it is aware of casts a spell of 6th level or lower, it forces that creature to face the effects of their own spell. Otherwise this reaction functions like a 9th level Counterspell.

Ruin (Costs 4 Stellar Charges). Yuxxilpoyx targets a creature within 500 ft of itself that it has line of sight to. It then expends a leftover 6th or 7th level spell slot and uses the attack as a single thin beam of unrecognizable color shoots towards the target. The target must make a constitution saving throw against Yuxxilpoyx's spell save DC or take 20d6 force damage which ignores resistances, obliterates force barriers, bypasses the shield spell, and breaks mage armor. Succeeding the saving throw only prevents the disabling of mage armor and allows the target to take half damage. A creature killed by this attack is reduced to a pile of dust along with everything on them except for legendary and artifact magical items.

Expel to the Far Realm (Costs 200 hp and 12 Stellar Charges). The targeted creature that Yuxxilpoyx is aware of within 1000 ft of it must make a DC 18 flat d20 check. Failure rips the target out of their plane and into the Far Realm for 1 day. Upon returning, the creature gains a permanent madness effect chosen by the DM and a short-term madness effect that lasts for 1 day as chosen by the DM.