Aberrant Ground. The ground in a 100-foot radius around the Fleshhemoth is a dough like difficult terrain. Each creature that starts its turn in the area must succeed a DC 20 Strength save or have its speed be 0 until the start of its next turn.

Mass of Mouthers. Every hit point the Fleshhemoth loses to slashing, lightning or thunder damage, it summons one Gibbering Mouther controlled by the Fleshhemoth.

Spellcasting. The Fleshhemoth is a 1st level spellcaster. Its spellcasting ability is Charisma (spell save DC 25, +10 to hit with spell attacks). The Fleshhemoth has following warlock spells prepared:
Cantrips (at will): Eldrich Blast, Mind Sliver
Note: the Fleshhemoth does not use normal spellist for the following spells, requiring no material components: 

9th level (5 slots): Gate, Wish, Psychic scream, mass polymorph

multiattack. the Fleshhemoth makes 1d6 bite attacks

Bite. Melee Weapon Attack: +15 to hit, reach 15ft., 1-10 targets. Hit: 36 (4d12 + 10) acid damage.

The Fleshhemoth has unlimited legendary actions

Bite. Fleshhemoth uses its bite attack.