Behemoth

Telegraphed. The Behemoths attacks are not made normally. The Behemoths attacks are instead Dexterity Saving Throws, representing their telegraphed movements allowing characters to dodge out of the way. However, if they fail, no armor can save them. These attacks are still considered as attacks despite being Saving Throws.
At the start of any other creatures turn, check if they have been damaged by the Behemoth since the end of their last turn. If they have not, then they gain Advantage on their first attack that targets the Behemoth until the end of their turn.

Aggression. Whenever a creature successfully deals 40 or more damage or applies an inhibiting effect to the Behemoth, that creature gains the Behemoths Aggression. Only one creature bears the Behemoths aggression at a time.
The Behemoth favors attacks that include the aggressor, often times exclusively targeting them.
At the start of a creatures turn, the Aggression effect on it ends if it had it. While a creature has the Behemoth's aggression, all other creatures have Advantage on Dexterity Saving Throws caused by the Behemoths actions, directly or indirectly.

Combination Strikes. The Behemoths telegraphed attacks flow in a combination. After resolving their Precise Impalement, Wide Cleave, or Brutal Crush attack actions, or the Grab action, from either direct resolution, or the Leap or Charge Special attack actions, that action option is no longer available to them until the Behemoths action options are 'refreshed'.
In a second wording, after taking any of those action options, that action option becomes unavailable until otherwise stated. A combination option means both the movement and attack option is considered to be used, becoming unavailable.

Recovering Respite. When the Behemoths turn starts and it has no more action options left to use, their turn is passed and they then regain all action options.
Additionally, after the Behemoths turn is passed, all healing and hit point recovery performed and resolved gains a bonus. Roll healing dice a second time, and add half of the result (rounded up) to the total healed. This condition lasts until the start of the Behemoths next turn.
Also additionally, during the instance of the Behemoths turn being passed, all other creatures can trade initiative values as long as both characters concede to the trade. Then continue initiative as normal, with the new order in mind.
Variant Trait Rule: A random amount of a variety of healing potions appear in a 60 foot radius around the Behemoth when this triggers.

Execute Attack. Enter the Resolution phase, where the Behemoth resolves their readied attack, causing saving throws, resulting damage, and any movement option.
Then enter the Preparation phase, where the Behemoth readies another action from this list, within any option restrictions imposed.
Readying an action is replaced with this action.

Movement. The Behemoth moves up to 40 feet, and loses preparation of their readied action. It ignores difficult terrain unless half of itself is on it, and when it ends moving, all creatures in its new space are moved to the nearest unoccupied space.
As the Behemoth can (possibly) always use this option, this is not a valid consideration when looking at what action options the Behemoth has taken already.

Readied Attack Actions.

Precise Impalement. Melee Weapon Attack: DC 16 Dexterity Saving Throw, targets a line 15 feet long and 5 feet wide.
Resolution: 20 (6d6) Piercing damage.
Creatures that failed this Saving Throw unanimously get pushed 25 feet away from the Behemoth.
The Behemoth can then move 15 feet in the direction of the line.
This attack is a readied attack.

Wide Cleave. Melee Weapon Attack: DC 12 Dexterity Saving Throw, 15 foot cone. 
Resolution: 20 (4d8) Slashing damage.
Creatures that failed this Saving Throw unanimously get thrown 4d4 x 5 feet to either the left or the right of the Behemoth. The Behemoth can then move 10 feet in the direction of the cone.
This attack is a readied attack.

Brutal Crush. Melee Weapon Attack: DC 14 Dexterity Saving Throw, 15 foot radius centered on a point 15 feet away from the Behemoth.
Resolution: 20 (2d12 + 6) Bludgeoning damage.
Creatures that failed this Saving Throw are knocked 2d4 x 5 feet away from the center of the Radius.
The Behemoth gains no base movement from this attack.
This attack is a readied attack.

Grab. The Behemoth targets the creature with the Behemoths aggression, if it is within 20 feet of the Behemoth. If another creature earns the Behemoths Aggression, the Behemoth is now targeting them if they are within 20 feet. Otherwise, it continues to target the same creature.
This attack is a readied attack.
Resolution: The target creature must a DC 18 Dexterity Saving Throw. On a failure, the creature is Grabbed. A grabbed creature cannot attack with weapons that have the two handed or heavy properties, or cast spells.
While grabbing a creature, the Behemoth can take the Brutalize Action, forcing the creature to make a DC 18 Constitution Saving Throw or take (50) 10d8 (Bludgeoning/Piercing/Slashing) damage, half on a successful save. The creature is then thrown into an unoccupied space 60 feet away from the Behemoth, and takes no falling damage from this.

Special attack options.

Leap. The Behemoth targets a space within 60 feet and leaps ((2d8 + 4) x 5) feet into the air, halfway to their chosen destination, and can additionally ready another attack action above, with altered effects, and does not get the movement options they grant.
Upon landing, all creatures in the Behemoths new space are moved to the nearest unoccupied space unless stated otherwise by any feature or ability.
On resolution:
Plunging Impalement resolves, targeting only one creature in the Behemoths new space, and additionally renders that creature Incapacitated for as long as the Behemoth keeps steady. The Behemoth acts as if they are concentrating, and on a failed concentration check the victim creature is no longer incapacitated, and is then moved to the nearest unoccupied space. When the Behemoth is moved, or resolves an action, the victim creature is also freed and moved.
Penalty: All attacks that successfully hit the Behemoth while they have rendered a creature incapacitated get a damage bonus equal to the amount of damage dice rolled, and the effect of any extra numerical damage values added.
This uses Impalement and Leap.
Slamming Cleave resolves, targeting a line 10 feet wide and 30 feet long from the Behemoths new space, and creatures get knocked to the side from the attacks area according to which side of the line they were on.
Penalty: Then, all attacks automatically hit the Behemoth until the start of its next turn.
This uses Cleave and Leap.
Ground Pound resolves, targeting a 10 foot radius around the Behemoth in their new space.
Penalty: The Behemoth then replaces their resistance of a damage type with a vulnerability until the start of its next turn. 
Roll a d6. On a roll of: 1-2, Bludgeoning Damage. 3-4, Piercing Damage. 5-6, Slashing Damage.
This uses Crush and Leap.

Charge. The Behemoth targets a space within 40 feet, and can additionally ready another attack action above with altered effects, and does not get the movement options they grant.
On resolution, the Behemoth moves in a directly straight line to that targeted space. Any creatures within the movement path of the Behemoth are moved to the nearest unoccupied space to the side and must make a DC 16 Dexterity Saving Throw or take 5d4 bludgeoning damage.
Then the readied action is resolved.
Charging Impale causes the first creature to take damage from Dash to suffer an additional (20) 5d6 Piercing damage, and be moved in front of the Behemoth.
Penalty: The Behemoth cannot ready an attack action or take the Execute Attack action on their next turn.
This uses Impale and Charge.
Charging Cleave causes all creatures along the Behemoths movement path to take (26) 2d8 + 16 Slashing damage instead of the normal 5d4 Bludgeoning damage if they failed the Charge Saving Throw.
Penalty: All attacks that successfully damage the Behemoth gain a bonus to damage equal to four times the amount of attacks that have already targeted the Behemoth since this was resolved, until the start of the Behemoths next turn.
This uses Cleave and Charge.
Charging Stomp causes all creatures that failed the Charge saving throw to take an additional (10) 2d4 + 4 Bludgeoning damage.
Penalty: The Behemoth cannot choose a special attack action when next picking an action, and deals half damage on the next resolution of damage dealt.
This uses Crush and Charge.

Aggravate. After taking any damage, if any other creature has the Behemoths aggression, the Behemoth can use its reaction to shift its aggression to the creature that damaged it, regardless of the amount of damage suffered.

Grand Movements. When a creature moves 20 feet or more, or takes an action and then moves 5 feet within 15 feet of the Behemoth, the Behemoth can take this reaction to force that creature to make a DC 12 Dexterity Saving Throw, dealing 4d4 Bludgeoning damage on a failure. If the Behemoth took the Movement action as its last action, and took this reaction, this reaction can trigger two more times until the start of its next turn.

At the end of any creatures turn, including the Behemoths, the Behemoth can take a Legendary action. The Behemoth has a total of 5 Legendary actions, which refresh at the start of its turn. The Behemoth can only use each legendary action once in each round of combat.

Instant Execute (Costs 3 Actions). The Behemoth instantly resolves their readied attack action/s.

Feint (Costs 2 Actions). The Behemoth cancels their current readied action if it is not the Leap or Charge actions, or one of the attack variations it brings, changing to a different available action. The action they falsely telegraphed is considered unavailable until the Behemoths options are refreshed.

Lunging / Retreating Attack (Costs ? Actions). The Behemoth can spend any amount of legendary actions, adding an extra 10 feet of movement per Legendary action spent to move them either towards or directly away from the direction of their readied attack, to resolve at the end of the resolution step.

Legendary Attack (Costs 4 Actions). The Behemoth instantly resolves their readied attack, applying disadvantage to all creatures making their saving throw. Then, the Behemoth can ready another action, but they lose their damage resistances until they resolve the action readied in this way, or instead take the Movement action as their next action.

Flurry (Costs 4 Actions (1/day)). The Behemoth refreshes their action options, and can then ready up to two attack actions, which resolve in order from the same execution. This does not allow the Behemoth to perform the same movement action twice, or the same attack twice.

Double Strike (Costs 3 Actions (Recharge - 6)). If the Behemoth has an attack readied, it readies an additional attack from any options available to them excluding special attack options, which resolves immediately after its prior prepared attack.

React (Costs 1 Action). The Behemoth regains its reaction.

Lead the Target (Costs 1 Action). The Behemoth changes the target area of its prepared attack/s.