Zuggtmoy, the Rotting Queen

Legendary Resistance (3/Day). If Zuggtmoy fails a saving throw, she can choose to succeed instead.

Vengeful Infection. Whenever Zuggtmoy loses 10% of her total health rounded up, she releases a cloud of spores within 30 feet of her for a number of rounds equal to the number of the phase she's in. When the cloud appears and whenever a creature starts its turn in it, a creature in the cloud must make a DC 23 Constitution saving throw or become infected with a disease. A creature infected with this disease is poisoned and takes 4d6 poison damage and loses 1d6 Constitution at the start of their turn.

Innate Spellcasting. Zuggtmoy's spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). Zuggtmoy can innately cast the following spells, requiring no components:
At will: thorn whip (17th level), blight, contagion
3/each day: cloudkill, feeblemind, wall of thorns

1/each day: psychic scream, time ravage

Magic Resistance. Zuggtmoy has advantage on saving throws against spells and other magical effects.

Regeneration. Zuggtmoy regains 20 hit points at the start of her turn. If she takes fire damage, this trait doesn't function at the start of her next turn. 

Born of Decay. Whenever a creature dies within 120 feet of Zuggtmoy, a fungal corpse flower immediately blossoms from the body.

Multiattack. Zuggtmoy makes three rotting whip attacks, then uses Fungal Command if available.

Rotting Whip. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 4d6 + 7 necrotic damage. On a hit, the target must make a DC 23 Constitution saving throw or be aged a number of years equal to a tenth of their lifespan. While a creature is aged this way, for each time they have been aged, they get a -1 penalty to their saving throws. If a creature cannot die of old age, they still begin to show signs of decay. This aging effect can be removed with greater restoration or wish.

Fungal Command (4-6). Zuggtmoy creates a corpse flower in an unoccupied space she can see within 30 feet of her, and it can use its reaction to either immediately make an attack against a creature within 10 feet of it or drop a corpse out of its body.

Obey Your Queen. All creatures within 120 feet of Zuggtmoy must make a DC 23 Wisdom saving throw or be charmed by her. While a creature is charmed by her this way, it views her as its trusted ruler, and defends her to the death. A creature charmed this way can make the saving throw again whenever they take damage from an enemy.

Zuggtmoy 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. Zuggtmoy regains spent legendary actions at the start of its turn.

Attack. Zuggtmoy makes two rotting whip attacks.

Blight (Costs 2 Actions). Zuggtmoy casts blight.

Hallucinatory Spores (Costs 2 Actions). Each creature within 15 feet of Zuggtmoy must succeed on a DC 23 Constitution saving throw. On a failure, a creature is poisoned. As long as a creature is poisoned this way, they are incapacitated.

Phase 1 - 750

Zuggtmoy begins her attack on the party, her shifting fungal dress and form flowing with power.

Field of Fungus. Zuggtmoy creates a field of writhing fungus around herself that extends 15 feet from her. Any creature that moves into it or ends its turn in it must succeed on a DC 23 Strength saving throw or be restrained by the fungus.

Animating Spores (2 Actions). Zuggtmoy releases a cloud of spores onto the corpses dropped by her corpse flowers within 30 feet. Each corpse is raised as a spawn of kyuss. Any undead raised by this effect can immediately make an attack against a creature within 5 feet of them.

Phase 2 - 525

Zuggtmoy empowers her fungi and spores and spreads her corruption further. A creature restrained by Field of Fungus takes 6d6 poison damage at the start of their turn.

Stench of Decay. Each creature within 30 feet of Zuggtmoy that can smell must make a DC 23 Constitution saving throw at the start of their turn. On a failure, the creature cannot use its reaction or regain hit points until the start of their next turn. A creature with Keen Smell or a similar feature has disadvantage on this saving throw.

Tiring Spores (Costs 2 Actions). Zuggtmoy releases spores that drain the energy out of the creatures they touch. Each creature within 30 feet of Zuggtmoy must make a DC 23 Constitution saving throw or gain a level of exhaustion.

Phase 3 - 350

Zuggtmoy begins absorbing her minions in fear of defeat. Animating Spores and Hallucinatory Spores now both cost 1 legendary action, and whenever a creature gains a level of exhaustion from Tiring Spores, Zuggtmoy regains hit points equal to the creature's number of hit dice.

Visions of Rot (Costs 2 Actions). Each incapacitated creature within 120 feet of Zuggtmoy loses 1d6 Wisdom and takes that many number of d12's of psychic damage.

Infectious Takeover (Costs 3 Actions). A creature within 30 feet of Zuggtmoy that is poisoned must make a DC 23 Constitution saving throw. On a failure, that creature's flesh begins to be consumed by fungus. At the end of each of their turns, they take 10d10 necrotic damage. If they drop to 0 hit points while affected by this, they immediately rise up as a spore servant of its creature type.

Phase 4 - 175

Zuggtmoy finally breaks and continually releases spores in fury. Field of Fungus now extends 60 feet out from her, Infectious Takeover now deals damage at the start of a creature's turn as well, and she uses one of her Spores legendary actions at the start of her turn at random.

Bow Down and Submit (Costs 3 Actions). Each creature who is poisoned treats immunity to being charmed as resistance, and resistance to being charmed as normal until the end of their next turn.

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