Ambrosia, the Thornmother of Ash and Rot

Symbiotic Entity (Recharges after Short of Long Rest). As a bonus action, Ambrosia awakens the fungal network within her body, gaining 100 temporary hit points and causing her melee attacks to deal +9 (2d8) necrotic damage. The effect lasts 10 minutes or until the temporary hit points are gone.

Rite of Thorns (1/Day). Ambrosia sacrifices 40 hit points to unleash a wave of sangromantic nature. In a 60-foot radius, ground and plants become difficult terrain. At the start of her turn, enemies in the area take 21 (6d6) piercing + 21 (6d6) necrotic damage (DC 17 Con save for half).
A creature that dies here rises as a spore servant under Ambrosia's control for 1 hour.

Sangromancy: Blood for Bloom. When Ambrosia casts a spell that deals necrotic damage or restores hit points, she may sacrifice up to 20 HP. For every 5 HP spent, add +1d8 to that spell's damage or healing.

Blighted Regrowth. When a creature dies within 30 ft. of her, she regains 10 hit points as fungal threads feed upon its remains.

Aura of Decay. Each creature of Ambrosia's choice that starts its turn within 20 ft. must succeed on a DC 17 Con save or take 10 (3d6) poison damage and have disadvantage on Strength checks until the start of its next turn.

Spellcasting. Ambrosia is a 14th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 17, +9 to hit with spell attacks). Ambrosia has following Druid spells prepared:
Cantrips (at will): Chill Touch, Thorn Whip, Druidcraft, Infestation
1st-5th level (4/3/3/3/2 slots): Entangle, Cure Wounds, Hold Person, Blight, Revivify (corrupted), Wall of Thorns, Vampiric Touch, Cloudkill, Insect Plague, Greater Restoration (Twisted to inflict decay instead of heal). 

Multiattack. Ambrosia makes two attacks with her Thorned Staff or casts a cantrip and makes one melee attack.

Thorned Staff. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 11 (1d8 + 7) bludgeoning damage.

Infestation Burst (Recharge 5-6). A cloud of toxic spores erupts in a 30-ft. radius centered on Ambrosia. Creatures in the area must make a DC 17 Con save, taking 36 (8d8) poison damage on a failure or half on success. Those who fail also hallucinate for 1 minute (as confusion).

Bloom Bloom (1/Day). Ambrosia slashes her palms and sends pollen and blood spinning though the air. Each enemy within 30 ft. takes 45 (10d8) necrotic damage (DC 17 Con save for half). Ambrosia and her allies heal half that amount.

Fungal Shield (3/Day). When Ambrosia is hit by an attack, she releases a burst of defensive mycelium, adding +4 AC until the start of her next turn.

Ambrosia can take 3 legendary actions, choosing from the options below. Only one may be used at a time and only at the end of another creature’s turn. She regains spent actions at the start of her turn.

Creeping Thorns. Vines lash at one creature within 30 ft. DC 17 Dex save or take 13 (3d8) piercing damage and be restrained.
Spore Bloom. A 10-ft. radius area within 60 ft. erupts with necrotic spores; creatures must make a DC 17 Con save or take 10 (3d6) poison damage.
Drain the Living (Costs 2 Actions). Ambrosia targets a creature within 30 ft. that is bleeding or dying; it takes 18 (4d8) necrotic damage and Ambrosia regains that many hit points.

Mythic Action Name. Enter the description for how mythic actions work for your monster here.

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.