Elder's Presence. Other plants within 60 feet of the treant are affected by the speak with plants spell and have advantage on saving throws while they remain in the area. The treant, along with each affected creature, can also speak with beasts as in the speak with animals spell.

False Appearance. While the treant remains motionless, it is indistinguishable from a normal tree.

Siege Monster. The treant deals double damage to objects and structures.

Spellcasting. The treant is a 15th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 19, +11 to hit with spell attacks). It has the following druid spells prepared:

Cantrips (at-will): druidcraft, guidance, razor leaf (new), resistance, tend plants (new)
1st level (4 slots): animal friendship, detect magic, entangle
2nd level (3 slots): animal messenger, barkskin, lesser restoration, spike growth 
3rd level (3 slots): conjure animals, dispel magic, plant growth 
4th level (3 slots): dominate beast, polymorph
5th level (2 slots): awaken, mass cure wounds, wrath of nature*
6th level (1 slot): druid grove, force of nature (new), wall of thorns 
7th level (1 slot): mighty roots (new),
8th level (1 slot): earthquake

Multiattack. The treant makes two slam attacks.

Slam. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 22 (4d6 + 8) bludgeoning damage.

Rock. Ranged Weapon Attack: +14 to hit, range 90/270 ft., one target. Hit: 35 (5d10 + 8) bludgeoning damage.

Animate Trees (3/Day). The treant magically animates one or two trees it can see within 60 feet of it. These trees have the same statistics as a treant, except they have Intelligence and Charisma scores of 1, they can't speak, and they have only the Slam action option. An animated tree acts as an ally of the treant. The tree remains animate for 1 day or until it dies; until the treant dies or is more than 120 feet from the tree; or until the treant takes a bonus action to turn it back into an inanimate tree. The tree then takes root if possible.

The treant can only have up to two trees animated under its power at once.

The treant 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. The treant regains spent legendary actions at the start of its turn.

Detect. The treant makes an Intelligence (Nature) check, a Wisdom (Perception) check, or a Wisdom (Survival) check.

Heal Flora or Fauna. The treant restores up to 15 hit points to one beast or plant that it can see within 60 feet. If the target is a plant that is on fire, that fire is magically doused.

Hurl Rock (Costs 2 Actions). The treant makes a rock attack, ignoring the disadvantage for ranged attacks in close combat.