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.

Spellcasting. The zombie is a 1st-level spellcaster. Its spellcasting ability is Charisma (spell save DC 12, +4 to hit with spell attacks). The zombie has following spells prepared:
Cantrips (at will): fire bolt, toll the dead
1st level (1x/day each): dissonant whispers, screams of the dead (tasha's hideous laughter)

Multiattack. The zombie makes a slam attack and a bite attack.

Slam. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 6 (1d6 + 3) bludgeoning damage.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 5 (1d4 + 3) piercing damage, plus 7 (2d6) necrotic damage.