Amphibious. The zamtrios can breathe air and water.

Multiattack. The zamtrios makes two Bite attacks.

Bite. Melee Weapon Attack: +6 to hit, reach 20 ft., one target. Hit: 16 (3d8 + 3) piercing damage. 

Ice Armor. While the zamtrios is Huge, it can coat its body in ice, increasing its AC by 5 for 1 minute.

Ice Drill. If the zamtrios ice armor is active and the zamtrios is underground it can then use this action to unburrow in a space occupied by another creature. The creature is pushed 5 feet out of the zamtrios space and must succeed on a DC 14 Dexterity throw or be knocked prone and take 13 (3d6+3) slashing damage plus 13 (3d6+3) piercing damage. On a successful save, the creature takes only half the damage, and isn't knocked prone.

Water Torrent (Recharge 5-6). The zamtrios fires a high-pressure beam of water across a 30-foot cone. Each creature in that area must make a DC 14 Dexterity saving throw, taking 24 (7d6) acid damage on a failed save, or half as much damage on a successful one.

Expand (2/day). The zamtrios greatly expands its belly for 1 minute. While expanded, the zamtrios is Gargantuan, has resistance to bludgeoning, piercing and slashing damage, it can no longer burrow, and its speed is reduced by 10 feet. If the zamtrios lacks the room to become Gargantuan, it attains the maximum size possible in the space available. The zamtrios can repeat this bonus action to revert back to its original size.