Aionar 'The Father of the Forest'

Heart of Mistletoe (Mythic Trait; Recharges after a Short or Long Rest).  If Aionar is reduced to 0 hit points, he doesn't die or fall unconcious. Instead he sheds his current form and takes on the form of a sprig of Mistletoe caught in a swirling cloud of wind and foliage. He regains 200 hit points and gains 100 temporary hit points. His AC becomes 22, his size becomes large, he loses his termorsense and he gains a flight speed of 60ft.

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

Siege Monster. Aionar deals double damage to objects and structures.

Speech of Beast and Leaf: Aionar can communicate with beasts and plants as if they shared a language.

Spellbark. Aionar's bark is engraved with Druidic wards, designed to repel dangerous spellcasting. Aionar has advantage on saving throws against spells and other magical effects. In addition, if he succeeds a saving throw, and the spell is of 3rd level or lower, the spell has no effect on Aionar, and the spell instead targets the caster. (DC 15) 

Innate Spellcasting. Aionar's is an 14th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 18, +10 to hit with spell attacks). He has the following druid spells prepared:
Cantrips (at will): Druidcraft, Mold Earth, Gust
1st level (4 slots): Beast Bond, Cure Wounds, Earth Tremor, Entangle
2nd level (3 Slots): Animal Messenger, Spike Growth, Warding Wind
3rd level (3 slots): Dispel Magic, Erupting Earth, Plant Growth, Wind Wall
4th level (3 slots): Conjure Woodland Beings, Grasping Vine, Hallucinatory Terrain
5th level (2 slots): Reincarnate, Wrath of Nature
6th level (1 slot): Heal, Wall of Thorns
7th level (1 slot): Whirlwind

Multiattack. Aionar makes two attacks: a rock attack and a slam attack.

Slam. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 20 (3d6 + 8) bludgeoning damage and if the target is a Large or smaller creature it must succeed on a DC 19 Strength saving throw or be knocked prone.

Rock. Ranged Weapon Attack: +14 to hit, range 60/180 ft., one target. Hit: 32 (4d10 + 8) bludgeoning damage.

Animate Guardians (Recharge 5-6). Aionar magically animates 2 trees that it can see within 60 feet of it. These trees have the same statistics as a Tree Blight. But have Vulnerability to Fire Damage. A Guardian acts as an ally of Aionar. The tree remains animated for 1 day or until it dies, Aionar dies or until Aionar takes a bonus action to turn it back into an inanimate tree. The tree then takes root if possible.

Aionar can take legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Aionar regains spent legendary actions at the start of its turn.

Attack. Aionar makes one rock attack or slam attack.

Thorns of the Forest (Costs 2 Actions). Aionar causes spiked roots to erupt from the ground in a 30-foot radius centered on him. The area becomes difficult terrain until the start of his next turn. Any creature, other than Aionar, takes 5 (2d4) piercing damage for every 5 feet it moves on those spikes.

Command Guardian (Costs 3 Actions). Aionar commands 1d4 Guardians which have been animated by his "Animate Guardians" action within 120 feet of him to immediately make a branch attack as a reaction.

If Aionar's mythic trait is active, he can use the options below as legendary actions for 1 hour after using Heart of Mistletoe.

Rampage. Aionar makes two slam attacks

Incantation of the Zephyr (Costs 2 Actions). Aionar casts one spell or cantrip provided they have the remaining spell slots to do so.

Summon the Swarm (Costs 2 Actions). Aionar summons 1d4 Swarm of Rot Grubs which appear in spaces adjacent to his prior Treant form - as they crawl from their host's body. Rot Grub Swarms act as allies to Aionar.