Cell Division. When Zac drops to 0 hit points, he divides into 4 gelatinous cubes with the following changes: they have a quarter of the normal health, and their size is medium. These cubes will try to merge together, reforming Zac. Any cubes that manage to merge together revive Zac with as many hitpoints as the gelatinous cubes had when they merged. Zac can only be revived in this way once per day.

Whenever Zac is hit with a critical attack, a piece of him falls off into a random open space within fifteen feet of Zac. This space counts as difficult terrain for everyone except Zac. If Zac steps into a space that has a lopped-off piece of himself, he regains 17 (3d10) hit points.

Unstable Matter. Whenever Zac is hit with an attack, all creatures within 5 ft of Zac take 17 (3d10) acid damage. 

Ooze. Zac can squeeze through any space at least an inch wide. 

Goopy. Zac can cast mending at will, sticking items together with slime. 

Multiattack. Zac makes three slam attacks, or two slam attacks and one tug attack. 

Slam. Melee Weapon Attack: +11 to hit, reach 10 ft., 1 target. Hit: 23 (4d8 + 5) bludgeoning damage and 14 (3d8) acid damage and the target is attached.

Tug.  If Zac attaches two targets during the same turn, he can pull both targets towards himself, slamming them together. Each target must make a DC 20 Strength saving throw, resisting the effect on a success and no longer being attached. If the target fails, they are tugged within 5 ft of Zac. If both targets fail, they are both pulled within 5 ft of Zac and are slammed into each other, taking 38 (5d10+10) bludgeoning damage. Both targets are left prone after the damage is taken. After Tug is used, all targets involved are no longer attached.

Let's Bounce! (Recharge 5-6). Zac leaps into the air and bounces down four times. This movement does not provoke opportunity attacks. Zac chooses a place 15 ft from where he currently is to land, then picks the same or another spot 15 ft from his current location to jump to. Each time Zac lands, each creature 10ft from Zac must make a DC 20 Strength saving throw or take 28 (5d10) bludgeoning and 28 (5d10) acid damage and being knocked prone, taking half damage and not being knocked prone on a success. 

Zac is a legendary creature that has three legendary actions. These actions can only be taken at the end of another creature's turn. Zac regains all spent legendary actions at the start of his turn.

Slam (Costs 1 Action). Zac makes a Slam attack.

Slingshot (Costs 1 Action). Zac launches himself like a slingshot to a space within 50 ft of himself. 

Bounce! (Costs 3 Actions). Zac can roll to try and recharge his Let's Bounce attack.