Abomination's Ruinous Physique. The Abomination regains 30 hit points at the start of its turn. If the Abomination takes radiant damage or suffers an effect that blocks healing, it regains only 10 hit points at the start of its next turn. 

Non-Radiant damage can only reduce the Abomination to 1 hit point, and if damage reduces the Abomination to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken. On a success, the Abomination drops to 1 hit point instead.

Abominous Resistance. Whenever the Abomination would make a saving throw against a spell or another magical effect it can choose to make a Constitution saving throw instead of whatever saving throw the effect calls for.

The Waking Flesh. When the Abomination fails a saving throw it may sacrifice one of its Grasping Tentacles and succeed instead. The Abomination cannot use this feature if it has no Grasping Tentacles, but it regains 1 lost tentacle at the beginning of every other one of its turns.

The disconnected tentacle falls into an unoccupied space within 5 feet of the Abomination and becomes a Wriggling Biter.

Magic Weapons. The Abominations weapon attacks count as magical.

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

Multiattack. The Abomination makes two Brutish Claw attacks and a Grasping Tentacle or Slavering Jaw attack, if they are available. 

Brutish Claws. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 23 (3d10+7) slashing damage and the target cannot gain the benefits of the Dash, Disengage, or Dodge action until the start of the Abomination's next turn.

Grasping Tentacles. Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 17 (3d6+7) bludgeoning damage, and the target is Grappled (escape DC 20). While Grappled in this way a creature is Restrained. The Abomination has two Grasping Tentacles, each of which can grapple only one target.

Slavering Jaw (Recharge 5-6). Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 46 (6d12+7) piercing damage.

Throw. The Abomination throws a creature it has Grappled up to 60 ft. The creature takes 17 (3d6+7) bludgeoning damage when it reaches impact and falls Prone. If the Abomination throws one creature at another creature the target must make a DC 20 Dexterity saving throw or take the same damage the thrown creature takes and be knocked Prone.

Frightful Screech (5/Day). As a reaction to taking 20 or more damage in a single attack, the Abomination screeches, forcing all creatures within 60 feet of it to make a DC 19 Wisdom saving throw or be Frightened until the end of their next turn.

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

Claw. The Abomination uses its Brutish Claw attack.

Crashdown (Costs 2 Actions). The Abomination braces itself and jumps to an unoccupied space within 50 feet of it. All creatures within 10 feet of where it lands must make a DC 20 Strength saving throw or take 13 (2d12) bludgeoning damage and fall Prone.

Calamitous Parasites (Cost 2 Actions). The Abominations spews parasites from within its jaw at a creature within 15 feet of it. The target must make a DC 19 Constitution saving throw or be infected with parasites. Until the parasites are removed the target must remake the save at the end of each hour, gaining a level of Exhaustion on a failure. Creatures can gain a maximum of 5 stacks of Exhaustion from this effect, and while infected with parasites, a creature cannot gain the benefits of a short or long rest. The parasites can be removed over the course of an hour with a DC 19 Medicine check, or through the application of 6th level or higher magical healing.