Auquone

Amorphous. The ooze can move through a space as narrow as 1 inch wide without squeezing.

Corrosive Form. A creature that touches the Aquone or hits it with a melee attack while within 5 feet of it takes 22 (4d10) acid damage. Any nonmagical weapon made of metal that hits the ooze corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the ooze is destroyed after dealing damage.

The ooze can eat through 2-inch-thick, nonmagical wood or metal in 1 round.

Size Change. The Aquone can use its action to change its shape, to a maximum of gargantuan and a minimum of medium. Other than its shape, its other statistics don't change.

False Appearance. The Aquone is indistinguishable from a normal lake or puddle of water while it isn't attacking. It can also move like water.

Engulf. When a creature is in the space of the Auquone, that creature takes 55 (10d10) acid damage, and is grappled (escape DC 22) by the Aquone. Until the grapple ends, the creature is also restrained, and can't breathe. This grapple is different from the one in the Auquone's pseudopod action. At the start of the Auquone's turn, each creature grappled this way takes 88 (16d10) acid damage. This feature can only happen when it is large or bigger. When it is large, it can hold 2 medium or smaller creatures, when it is huge, it can hold 4 medium or smaller creatures or 2 large creatures. When it is gargantuan, it can hold 8 medium or smaller creatures or 4 large creatures or 2 huge creatures.

Multiattack. The Aquone attacks 4 times with its Pseudopod.

Pseudopod. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 18 (2d10 + 7) bludgeoning damage plus 33 (6d10) acid damage, and if the target is wearing nonmagical metal armor, its armor is partly corroded and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10. If the target is large or smaller, the Auquone can choose to grapple the target (escape DC 16). Any creature grappled is also restrained by the Auquone and takes 22 (4d10) acid damage at the start of the Auquone's turn. The Auquone can grapple up to 6 creatures. Any creature grappled by the Auquone can be pulled towards the Auquone, up to 10 feet a turn. Any creature that is pulled into the Auquone's space is engulfed by the Auquone.