Behemoth

Brute: A melee weapon deals three extra damage die of its damage when the behemoth hits with it (included in the attack).

Strong. The behemoth can hold a two handed weapon of its size in one hand without disadvantage on attack rolls.

Undead Fortitude. If damage reduces the behemoth to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from critical hits. On a success, the behemoth drops to 1 hit point instead.

Legendary Resistance (3/day). If the behemoth fails a saving throw, it can choose to succeed instead.

Magic resistance. The behemoth has advantage on saving throws against spells and other magical effects.

Rampage. When the behemoth reduces a creature, structure or object to 0 hit points with a melee or ranged attack on its turn, the behemoth can take a bonus action to move up to its movement speed and make a cleaver attack.

Death Burst. When the behemoth dies, it explodes in a burst of acid. Each creature within 30 feet of it must make a DC 20 Dexterity saving throw, taking 36 (8d8) acid damage, or half as much on a successful one. Objects or structures take double the damage and are melted into sludge if destroyed.

Acid Absorption. Whenever the behemoth is subjected to acid damage, it takes no damage and instead regains a number of hit points equal to the acid damage dealt, giving temp hit points for every point over max health.

Immutable Form. The behemoth is immune to any spell or effect that would alter its form.

Siege Monster. The Behemoth does double damage to objects and structures.

False Life. While the behemoth is at 1 hit point it still has half the damage taken in the round on it. if the damage does not exceed its regenerative ability, at the start of its turn, it regains hit points as normal, provided radiant damage was not used.

(E.G. The behemoth was dropped to 1 hit point by 30 piercing damage. 15 of that damage stays until the start of the next turn, and then it would regenerate. If more damage was done before the start of its turn, to equal 30 or more, the behemoth stays at 1 hit point.)

I'll Take That. If the behemoth severs, or tears off a limb or appendage the affected creature takes 21 (6d6) damage, at the start of their turn, due to blood loss, for each limb or appendage taken in this way. (i.e barbarian missing arm and leg takes (12d6) damage at the start of their turn.) This lasts until the the blood loss stops, and can only be stopped with a superior healing potion, greater restoration or a medicine check DC 25, separate for each limb. Only a regenerate or similar spell can regrow limbs or body parts lost in this way.

Multiattack. The behemoth makes two attacks with its cleaver, one with its meat hook and one "Get Over Here!" action on each of its turns.

Cleaver (Right Hand). Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 49 (6d12 + 10) slashing damage plus 10 (4d4) acid damage.

On a critical hit, roll a d100, on a result of 85-100 roll a die equal to the targets+1 limbs, arms, legs, tail, head and wings. On a 1 on that die nothing happens, on any other number roll another melee attack. If this melee attack hits the target as well a limb chosen by the DM is severed, max on the die is always the head. Target then takes damage based on "I'll take that."

Hook. RangedWeapon Attack: +13 to hit, range 60 ft.,  one creature. Hit: 37 (6d8 + 10) piercing damage plus 10 (4d4) acid damage

If the target takes 45 or more damage from the attack they must make a DC 23 Dexterity saving throw, on a success nothing happens, on a failure the hook pierces into a random part of their body. The target is now hooked and considered grappled, and now susceptible to "Get Over Here!"

Horrifying Visage. Each non-undead creature within 80 feet of the Behemoth that can see him must succeed a DC 23 Wisdom saving throw or be frightened for 1 minute. A frightened target can repeat the saving throw at the end of each of its turns, with disadvantage if the Behemoth is within line of sight, ending the effect on itself on a success. If the target's saving throw is successful or the effect ends for it, the target is immune to the Behemoth's Horrifying Visage for the next 24 hours.

Get Over Here!. The Behemoth pulls a creature grappled by it's meat hook 25 ft. towards it. Anything hooked by the meat hook must make an Athletic or Acrobatics check against the behemoth's Athletics. On a fail the creature takes 65 (10d12) Piercing Damage, and is pulled all the way toward the behemoth and knocked prone. On a fail of 5 or more the target takes the damage and has the hooked appendage torn off, the creature now takes damage per "I'll Take That."

Acid spray (Recharge 3-6). When it with an attack that does slashing or piercing damage the behemoth may as a reaction spray acid in a 15 foot cone. creatures in the area must make a DC 20 Dexterity saving throw, and take 55 (10d10) acid damage, half as much on a save.

Objects and structures caught in the cone take double damage and are turned to sludge if dropped to 0 hit points.