Legendary Resistance (2/Long Rest). If Zac fails a saving throw, he can choose to succeed instead.
Amorphous Form. Zac can occupy another creature’s space and vice versa.
Corrode Metal. Any nonmagical weapon made of metal that hits Zac 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 Zac is destroyed after dealing damage.
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.
Zac can eat through 2-inch-thick, nonmagical metal in 1 round.
False Appearance. While Zac remains motionless, he is indistinguishable from Sump puddles in Zaun.
The Secret Weapon. Zac has the following benefits due to his nature as a Chemtech Ooze:

• Zac doesn’t require sleep and can move through a space as narrow as 1 inch wide without squeezing.
• Any creature that starts its turn in Zac’s space is targeted by a Stretching Strikes attack if he isn’t incapacitated.
• Goo: Unstable Matter, Elastic Slingshot, and Let's Bounce! leave one clump of goo within 10ft of the target creature, per creature hit. If Zac moves to a clump of goo he regains 9 (1d6 + 5). If another creature walks over this goo or deals damage to it, it is destroyed.
• Cell Division (1/Long Rest): When Zac is reduced to 0 hit points he doesn't die or fall unconscious. Instead he splits into 4 bloblets that are 15ft apart with 23 hit points each, which begin to move toward each other to rejoin. At the start of Zac's turn, all surviving bloblets combine their hit points to restore Zac (max of 112 hit points).
• Highly Flammable: If Zac is struck by an attack or spell which deals fire damage, he ignites. Zac takes 4 (1d6)fire damage at the start of his turn and deals extra fire damage with his attacks while ignited.

Multiattack. Zac makes three Stretching Strikes attacks.

Stretching Strikes. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 26 (4d8 + 6) bludgeoning damage plus 10 (2d8) acid damage. If Zac hits at least two creatures within 15ft. of each other he may attempt to slam them together. The creatures must succeed a DC 17 Strength saving throw or take an extra 4 (1d6) as they're pulled together and stunned until the end of their next turn.

Unstable Matter (Recharge 5-6). Zac expels a spray of his gelatinous mass in a 15-foot radius centered on himself. Each creature in that area must make a DC 15 Dexterity saving throw. On a failed save, the creature takes 24 (4d10) acid damage. On a successful save, the creature takes half as much damage.

Elastic Slingshot. Zac launches himself at a location or creature within a 90-foot cone from him.

Toasty. If on fire, Zac can invert himself to douse the flames inside himself.

Dull Acid. Zac can choose to spare any Engulfed creatures of his choice from taking acid damage.

Flex. Zac adds +3 to his AC until the start of his next turn.

Zac 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. Zac regains spent legendary actions at the start of his turn.

Move. Zac moves up to half his speed
Sticky Slap. Zac makes one Stretching Strikes attack.
Incoming! (Costs 2 Actions).  Zac uses Elastic Slingshot.
Engulf (Costs 2 Actions). Zac moves up to 20ft.. While doing so, he can enter Large or smaller creatures' spaces. Whenever Zac enters a creature's space, the creature must make a DC 15 Dexterity saving throw. On a successful save, the creature can choose to be pushed 5 feet back or to the side of Zac. A creature that chooses not to be pushed suffers the consequences of a failed saving throw.
On a failed save, Zac enters the creature's space, and the creature takes 12 (3d6) acid damage and is engulfed. The engulfed creature can't breathe, is restrained, and takes 21 (6d6) acid damage at the start of each of the cube's turns. When Zac moves, the engulfed creature moves with him.
An engulfed creature can try to escape by taking an action to make a DC 17 Athletics check. On a success, the creature escapes and enters a space of its choice within 5 feet of the cube.