Amorphous Form: The elemental can move through a space as narrow as 1 inch wide without squeezing.

Corrosive Aura: At the start of each of the elemental’s turns, each creature within 10 feet of it takes 21 (6d6) acid damage.

Legendary Resistance (3/Day): If the dreadnaught fails a saving throw, it can choose to succeed instead.

Magic Resistance: The dreadnaught has advantage on saving throws against spells and other magical effects.

Multiattack: The dreadnaught makes two attacks: one with its corrosive claw and one with its engulfing maw.

Corrosive Claw: Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 29 (4d8 + 9) acid damage.

Acid Breath (Recharge 5-6): The dreadnaught exhales acid in a 90-foot cone. Each creature in that area must make a DC 23 Dexterity saving throw, taking 77 (22d6) acid damage on a failed save, or half as much damage on a successful one.

Engulfing Maw: Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 32 (4d10 + 9) acid damage. If the target is a Large or smaller creature, it must succeed on a DC 23 Dexterity saving throw or be swallowed by the dreadnaught. A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the dreadnaught, and takes 42 (12d6) acid damage at the start of each of the dreadnaught’s turns. If the dreadnaught takes 50 damage or more on a single turn from a creature inside it, the dreadnaught must succeed on a DC 23 Constitution saving throw at the end of that turn or regurgitate the swallowed creature, which falls prone in a space within 10 feet of the dreadnaught. If the dreadnaught dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 30 feet of movement, exiting prone.

Enter the description for how bonus actions work for your monster here.

Reaction Name. Enter the description for your action.

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

• Detect: The dreadnaught makes a Wisdom (Perception) check.
• Corrosive Slam: The dreadnaught makes one corrosive claw attack.
• Acidic Burst (Costs 2 Actions): The dreadnaught releases a burst of acid in a 20-foot radius. Each creature in that area must make a DC 23 Dexterity saving throw, taking 35 (10d6) acid damage on a failed save, or half as much damage on a successful one.