Freedom of Movement. The yama tsukami ignores difficult terrain, and magical effects can't reduce its speed or cause it to be Restrained. It can spend 5 feet of Movement to escape from nonmagical restraints or being Grappled.

Siege Monster. The yama tsukami deals double damage to Objects and structures.

Multiattack. The yama tsukami makes three Tentacle attacks, each of which it can replace with one use of Fling.

Bite. Melee Weapon Attack. +14 to hit, reach 5 ft., one target. Hit: 21 (3d8 + 8) piercing damage. If the target is a Large or smaller creature grappled by the yama tsukami, that creature is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the yama tsukami, and it takes 42 (12d6) acid damage at the start of each of the yama tsukami's turns. If the yama tsukami takes 40 damage or more on a single turn from a creature inside it, the yama tsukami must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the yama tsukami. If the yama tsukami dies, a swallowed creature is no longer Restrained by it and can escape from the corpse using 15 feet of Movement, exiting prone.

Tentacle. Melee Weapon Attack. +14 to hit, reach 20ft., one target. Hit: 18 (3d6 + 8) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is Restrained. The yama tsukami has six tentacles, each of which can grapple one target.

Fling. One Large or smaller object held or creature Grappled by the yama tsukami is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 22 Dexterity saving throw or take the same damage and be knocked prone.

Fire Plume (Recharge 5-6). The yama tsukami releases a plume of fire from its underside in a 30-foot radius around it. Each creature in that area must make a DC 20 Dexterity saving throw, taking 28 (8d6) fire damage on a failed save or half as much damage on a successful one.

Summon Great Thunderbugs (Recharge 6). The yama tsukami releases 5 (1d10) great thunderbugs from its mouth that occupy a space within 20 feet of the yama tsukami. When a great thunderbug summoned by the yama tsukami dies, it explodes in a burst of lightning. Each creature within 10 feet of it must make a DC 13 Dexterity saving throw, taking 7 (2d6) lightning damage on a failed save, or half as much damage on a successful one.

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

Summon Great Thunderbugs or Fire Plume. The yama tsukami uses its Summon Great Thunderbugs or Fire Plume.

Tentacle Attack or Fling. The yama tsukami makes a tentacle Attack or uses its Fling.

Whirlwind (Costs 2 Actions). The yama tsukami extends out all of its tentacles and spins in circles. Each creature within a 20 foot radius of the yama tsukami must succeed on a DC 22 Dexterity saving throw, taking 22 (4d10) bludgeoning damage and are knocked prone on a failed save.