Ice Armor (Recharge 5-6).  When enraged, the Zamtrios can encase its body in durable, jagged ice as an action. This ice armor lasts for 1 minute, or until the Zamtrios takes 30 fire damage or more in a single round.

While ice armor is active, the Zamtrios' melee attacks deal an additional 1d4 cold damage, and its AC increases by 3.

Inflatable Hide. The Zamtrios possesses a unique physiology which allows it to inflate itself in response to heavy injuries. If an attack drops the Zamtrios' HP to 63 or below, the Zamtrios can use its reaction to become gargantuan in size. The Zamtrios gains 2d10+10 temporary hit points and resistance to bludgeoning, piercing and slashing damage. However, its AC drops by 2, its walking speed becomes 10 feet, and its swimming and burrow speeds become 0. The Zamtrios gains access to several unique abilities while in this inflated form, which replace its normal list of actions.

This transformation lasts for 1 minute, or until the Zamtrios uses its action to end it early, and cannot be used again until the Zamtrios finishes a short or long rest. When the Zamtrios exits this form, creatures within 10 feet must succeed a DC 15 Constitution Saving Throw or take 9 (2d8) cold damage and are pushed back 10 feet as it expels a freezing gas while it deflates.

Inflatable Hide and Ice Armor cannot be active simultaneously. If the Zamtrios chooses to inflate itself while its ice armor is active, the ice armor is broken.

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 6) piercing damage.

Claw. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 17 (2d10 + 6) slashing damage.

Charge. If the Zamtrios moves at least 20 feet straight toward a target and then hits it with a bite attack on the same turn, the target takes an extra 9 (2d8) bludgeoning damage. If the target is a creature, it must succeed on a DC 15 Strength saving throw or be thrown 5 feet away and knocked prone. Any creatures between the Zamtrios and the target during its charge must succeed a DC 13 Dexterity Saving Throw or take 4 (1d8) bludgeoning damage.

Water Torrent (Recharge 5-6). The Zamtrios shoots a beam of water from its mouth across a 30-foot cone centered on itself. Each creature in that area must make a DC 14 Dexterity saving throw, taking 21 (6d6) cold damage on a failed save, or half as much damage on a successful one.

Ice Drill. If the Zamtrios' ice armor is active and it burrows at least 10 feet straight towards a target, it can then use its attack action to unburrow in the target's space if they are within 5 feet. The target is thrown 10 feet away and must succeed on a DC 13 Dexterity saving throw or take 13 (2d6+6) piercing damage plus 7 (1d6+4) cold damage and be knocked prone. On a successful save, the creature takes only half the damage.

Inflated Form

Leaping Crush. Melee Weapon Attack: +9 to hit, reach 20 ft., one target. The Zamtrios leaps into the target's space, dealing 15 (2d8+6) bludgeoning damage and the target must make a DC 15 Strength Saving Throw or be grappled by the Zamtrios.

Rolling Crush. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. The Zamtrios rolls up to its walking speed in a straight line toward a creature it can see and smashes into them with the full force of its tremendous mass. Hit: 24 (4d8+6) bludgeoning damage, and the target must succeed a DC 15 Strength Saving Throw or be knocked prone. Any creatures between the Zamtrios and the target during its charge must succeed a DC 13 Dexterity Saving Throw or take 7 (2d6) bludgeoning damage.

Mass Cold Spit. Ranged Weapon Attack: +9 to hit, range 20/60 ft., three targets. Hit: 12 (3d6+2) cold damage, and the target must succeed a DC 15 Strength Saving Throw or be restrained in a ball of ice until the beginning of the Zamtrios' next turn.