Monstrous Resilience.
Once per day, when reduced to 0 hit points, the Mawbeast is instead reduced to 1 HP and becomes enraged until the end of its next turn. During that time, it has advantage on all attacks and saving throws.

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

Brutish Momentum.
If the Mawbeast moves at least 20 feet straight toward a creature and hits it with a slam on the same turn, the target must succeed on a DC 18 Strength save or be knocked prone and grappled (escape DC 18).

Towering Menace.
Creatures within 10 feet of the Mawbeast at the start of their turn must succeed on a DC 15 Wisdom saving throw or become frightened until the end of their turn.

Multiattack.
The Mawbeast makes three attacks: two slams and one bite.

Slam. Melee Weapon Attack: +12 to hit, reach 10 ft., one target.
Hit: 20 (2d10 + 9) bludgeoning damage.

Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one target.
Hit: 30 (4d10 + 8) piercing damage. If the target is Large or smaller, it is grappled (escape DC 18) and restrained. The Mawbeast can grapple only one creature this way.

Devour.
A creature grappled by the Mawbeast at the start of its turn must make a DC 18 Dexterity save or be swallowed. While swallowed, the creature is blinded and restrained, has total cover, and takes 28 (8d6) acid damage at the start of each of the Mawbeast’s turns. If the Mawbeast takes 35 or more damage in a single turn from a swallowed creature, it must succeed on a DC 18 Con save or regurgitate them at the end of that turn.

Crush.
The Mawbeast makes a contested Strength (Athletics) check against a creature it is grappling. On a success, it deals 18 (4d6 + 4) bludgeoning damage and knocks the target unconscious for 1 round if they fail a DC 15 Constitution saving throw.