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.

Baleful Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 8 (1d12 + 2) piercing damage. If the target is a creature, it must succeed on a DC 13 Wisdom saving throw or become charmed after 8 hours.

While charmed in this way, the target is compelled to return to the creature's grave and can repeat the saving throw after every 24 hours that elapse. On a successful save, the charm ends. On a failed save, the target’s hit point maximum is reduced by 4 (1d8). The target dies and rises as another Mourner's Mire if its hit point maximum is reduced to 0.

The Mourner’s Mire, a necromantic by-product of an incredibly toxic relationship that lingers even past death. It preys on grief, and hides itself in fresh grave dirt. Once the Mourner’s Mire senses something above it, it ensnares the victim using its jaw-like limbs, and wraps the prey in a toothy embrace. 

When the Mourner’s Mire bites a target it has a chance of leaving a lingering charm which only manifests after 8 hours. Even if the target escapes, they soon receive strong urges to visit the cemetery again. Perhaps that monster simply needed a hug, and it was all just a grave misunderstanding.