Agarus

Legendary Resistance (3/Day).  If an Agarus fails a Saving Throw, it can choose to succeed instead.
Legendary Multiattack (2/Day).  An enraged Agarus can use Bludgeoning Rock Slam twice in one turn.

Multiattack.  An Agarus can use its Vine Arm Whip attack twice in one turn.

Vine Arm Whip.  Melee Weapon Attack: An Agarus will wind up its thin, vine-like left arm, and swing at its targets like a whip.  If an opponent is hit by this, they must make a DC14 Dexterity Saving Throw, and if they fail they will be knocked prone.  +12 to hit, range 25ft., one target.  Hit: 22 (2d12 + 6) slashing damage.
Bludgeoning Slam.  Melee Weapon Attack: An Agarus, will use its massive hammer-like right arm, to slam the opponents that dare oppose it.  +12 to hit, range 10ft., up to three targets.  Hit: 36 (3d12 + 10) bludgeoning damage.
Flailing Sweep.  Melee Weapon Attack: In an effort to make a quick change of direction, an Agarus will swing its bludgeoning arm in a single direction to drag itself 180 degrees its original position before the attack.  +12 to hit, range 10ft., up to four targets.  Hit: 30 (2d12 + 10) bludgeoning damage.

Toxic Orb Throw.  Ranged Weapon Attack: The Agarus will grab Terra Aether off of its back, and throw it at one of its opponents, causing a 5ft. radius pool of poison to appear where it landed for one minute.  If this hits a target directly, they must make a DC18 Constitution Saving Throw, and if they fail they will be poisoned, taking 8 (1d8 + 4) poison damage for 15 seconds.  +2 to hit, range 15/60ft., up to three targets.  Hit: 6 (1d6 + 2) poison damage.

Stone Upheaval.  Aethercharged Ranged Attack: While an Agarus is Aethercharged, it can summon rocks that come shooting up into the air and the back down again, and hitting anyone who tries to smash its roots in.  +12 to hit, range 60ft., up to three targets.  Hit: 17 (1d12 + 9) bludgeoning damage.
Bludgeoning Rock Slam.  Enraged Melee Attack: While an Agarus is enraged, it can grab a large rock, and smash it down two times before it breaks from the force of it hitting the ground.  This attack can hit twice.  +12 to hit, range 10ft., any targets within 5ft. radius of the attack.  Hit: 24 (3d8 + 12) Bludgeoning damage.

Enrage.  An Agarus after taking considerable damage, will flail both its arms around, slamming and whipping them, and eventually enter its enraged status.  The Agarus gains new abilities, along with attacking harder (6 [1d6] extra damage for every single attack).  The Agarus will eventually grow tired in this state, taking one turn of combat to rest to un-enrage.

Aethercharge.  An Agarus that is on the verge of Aethercharging will move to the center of its arena, and then spread its four massive roots across the arena, and protecting itself inside a shield of Terra Aether.  The wooden horror is now Aethercharged.  In this state, it will heal every turn of combat for 4 (1d4 + 2) hit points, and the opponents must break its roots in order for it to leave this state.  Each root has 30 (2d20 + 5) hit points.  Once all the roots are destroyed, the Agarus will become immobile for three turns of combat to regain its energy.

Awakening.  If an opponent enters within 60 feet of the Agarus, then it can make a DC14 perception check with advantage, and on success it will awaken from its feeding harvest, retracting its large roots and prepare itself for battle.  Be careful to not be caught within the retracting process of the roots, as they can pack quite a punch.  +6 to hit, range 40ft., two targets.  Hit: 18 (2d8 + 8) bludgeoning damage.

Poisonous Slam.  If two or more opponents are attacking the Agarus' trunk, then it can choose to use its arms to lift itself up, and slam down on its attackers, leaving a poisonous pool around it's trunk for one minute.  +12 to hit, range 5ft., all targets within range.  Hit: 22 (3d8 + 4) bludgeoning poison damage.

An Agarus can take 3 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.  An Agarus will regain legendary actions at the start of it's turn.

Lively Roots (Costs 2 Actions).  An Agarus will stick its vine arm into the ground, and choose one to five targets to lift its roots up to, and steal their life.  The targets must make a DC20 Dexterity Saving Throw, and if they fail, they will be ensnared by the Agarus for one turn of combat, taking 12 (2d8 + 2) necrotic damage.  75% damage taken from the targets will be returned to the Agarus as health gained.
Branching Whip (Costs 1 Action).  An Agarus will select two targets by its thin and whip-like left arm, and swing it at them like a whip.  +12 to hit, range 25ft., two targets.  Hit: 16 (2d8 + 5) slashing damage.

Agarus are mythical creatures to those outside of the Shattered Isles as the Agarus, along with the other known Behemoths, are nothing the likes of which anyone has ever seen before.

Sheer Mystery. Anyone who is not from the Shattered Isles cannot seem to identify this creature, or what it can do, so people who aren't from the Shattered Isles are weary and unsure about the Behemoths. Creatures who have the Mythical Trait of Sheer Mystery are able to frighten those who have not encountered this creature previously for 2 turns of combat if the creature is huge or bigger.  If the creature is smaller than huge, the opponent must make a DC14 Wisdom Saving Throw, and if they fail they will be frightened for 2 turns of combat.