Bloated Colossus

Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Slippery Blood. Whenever a creature moves within or starts its turn within 5 ft of a bloated colossus that is below its hit point maximum, the creature must make a DC 15 dexterity saving throw or be knocked prone.

Rupture. When the bloated zombie is critically hit or dies everyone within 5ft that is not undead must make a DC 18 constitution saving throw or take 5d10 necrotic damage on a failed save, or half as much damage on a successful one. 

Multiattack. The bloated zombie makes two attacks

Slam. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (3d6 + 7) bludgeoning damage.