Legendary Resistance (3/day): When the Abominable Algrion fails a saving throw, it can choose to succeed instead.

Grapple: Once per turn, when the Abominable Algrion hits a creature with a bite attack, it can choose to grapple the target. The target must succeed a dc 26 dexterity saving throw. On a fail, the creature is grappled. The creature can repeat the save on each of its turns, freeing itself on a successful save. If the Abominable Algrion makes a bite attack against a creature that is not the creature it is grappling or if it uses its Malevolent Spew attack, the creature it is grappling is released.

Abominable Speed (3/day): Once per turn the Abominable Algrion can boost itself with unnatural speed. For the rest of its turn, its movement speed is doubled, its armor class is increased by 2, and it can take 1 additional action.

Multiattack (1/turn): The Abominable Algrion makes 2 claw attacks and 1 bite attack.

Claw: +18 to hit, 5 foot range, one target, 17 (2d6+10) slashing damage + 9 (2d8) necrotic damage. The target must succeed a dc 23 constitution saving throw. On a fail, the target's constitution score is permanently reduced by 1d4, to a minimum of 1. A casting of Greater Restoration can cure a creature's reduced constitution. A hit with this attack against a creature the Abominable Algrion is grappling is treated as a critical hit.

Bite: +18 to hit, 10 foot range, one target, 28 (4d8+10) piercing damage + 9 (2d8) necrotic damage. The target must succeed a dc 23 constitution saving throw. On a fail, the target's strength score is permanently reduced by 2d4, to a minimum of 1. A casting of Greater Restoration can cure a creature's reduced strength. 

Stomp: +18 to hit, 5 foot range, one target, 31 (6d6+10) bludgeoning damage + 18 (4d8) piercing damage. The target must make a dc 26 dexterity saving throw. On a fail, they are knocked prone and their speed becomes 0, preventing them from getting back up. If the Abominable Algrion moves or uses a stomp attack again, the target's speed returns to normal, allowing them to get back up on their turn.

Malevolent Spew (Recharge 5-6): Each creature in a 60 foot cone must make a dc 23 constitution saving throw. On a fail, they take 78 (12d12) necrotic damage, or half on a successful save. Additionally, creatures that fail the save become vulnerable to bludgeoning, piercing, and slashing damage and disadvantage on strength, dexterity, and constitution checks and saving throws until the end of the Abominable Algrion's next turn.

Rejuvenating Rot: The Abominable Algrion can use its bonus action to heal a number of hit points equal to the amount of necrotic damage it dealt with its bite and claw attacks during its current turn.

Dash: The Abominable Algrion can take a dash action as a bonus action.

Devour: When the Abominable Algrion kills a medium or smaller creature with its bite attack, it can use its bonus action to consume it, swallowing it and everything on it and healing itself a number of hit points equal to 22 (4d10) + the devoured creature's level (if its a player character) or challenge rating (if its a non-player character). The devoured creature and all nonmagical items on it are destroyed within 1 hour of being consumed.

Legendary Actions (3/day)

Claws (costs 1 action): Makes 2 claw attacks.

Bite (costs 1 action): Makes a bite attack.

Multiattack (costs 2 actions): Makes 2 claw attacks and 1 bite attack.

Malevolent Spew (costs 3 actions): Makes a Malevolent Spew attack, ignoring recharge time and not requiring recharge.