Limited Magic Resistance. Yixhgornu Rage has advantage on saving throws against spells and other magical effects at 2nd level or lower.

Legendary Resistance (3/Day). If the monster fails a saving throw, it can choose to succeed instead.

Three Heads. Yixhgornu has three heads, each with its own special properties. Yixhgornu must be killed thrice, one for each personality. Smile, Rage, and Brain. Brain is  his main head, the others are secondary personalities he created for his wild mood swings. Subsequent heads have reduced hp(76). At the end of each turn, he may switch heads freely. If a head is killed, he is forced to switch to another.

• Smile - Yixhgornu Smile uses Clerical magic and seldom uses his attacks, attempting to flee the group and buff his body for the more powerful other heads. He does not use Ice Beam.
• Brain - Yixhgornu Brain uses Arcane magic and his Ice Beam. He is likely to switch to another head if he feels cornered.
• Rage - Yixhgornu Rage deals an additional damage dice with all of his attacks and Ice Beam.

Spellcasting. Yixhgornu Smile is a 7th-level spellcaster. Yixhgornu Brain is a 9th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 16, +6 to hit with spell attacks). The heads share spell slots. Yixhgornu Smile has the following spells prepared:

Cantrips (at will): toll the dead

1st level (4 slots): inflict wounds, bane, detect magic

2nd level (3 slots): lesser restoration, enhance ability, blindness/deathness

3rd level (3 slots): sending, bestow curse, animate dead

4th level (3 slots): guardian of faith, freedom of movement

Yixhgornu Brain has the following spells prepared.

Cantrips (at will): encode thoughts, minor illusion

1st level (4 slots): identify, ice knife, feather fall, false life, charm person

2nd level (3 slots): snilloc's snowball swarm, locate object, detect thoughts, knock

3rd level (3 slots): counterspell, thunder step, slow

4th level (3 slots): ice storm, evards black tentacles

5th level (3 slots): cone of cold, dream, far step

Multiattack. Yixhgornu uses Ice Beam once if it is out of range. Otherwise, it then makes three attacks: one with its sword and two with its claws.

Claws. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 10 (2d4 + 5) slashing damage.

Greatsword. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 16 (2d10 + 5) slashing damage. 

Ice Beam. (Recharge 3-4) RangedWeapon Attack: +7 to hit, range 40/120 ft., one target. Hit: 23 (4d8 + 3) frost damage.

Quickened Spell. (Recharge 3-4) Yixhgornu Smile or Brain can use a cantrip in addition to their action.

The legendary monster can take 3 legendary actions. Only one legendary action option can be used at a time and only at the end of another creature's turn. The legendary monster regains spent legendary actions at the start of its turn.

Move (Costs 1 Action). Yixhgornu moves up to 30 feet.

Attack (Costs 1 Action). Yixhgornu makes an attack with his greatsword.

Ice Beam (Costs 2 Actions). Yixhgornu makes an attack with Ice Beam.

Ice Stomp (Costs 3 Actions). Yixhgornu creates spikes of ice all around him, dealing 11 (2d10) frost damage in a 10ft radius.