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.

Lightning Aura.  Each creature that starts its turn within 10 ft. of the zombie or hits it with a melee attack takes 2 (1d4) lightning damage.  

Multiattack.  The zombie makes three melee attacks.  It can replace one of them with Zap.

Shocking Slam. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage plus 6 (1d12) lightning damage.  The target can't take reactions and has its speed reduced by 10 ft. until the start of the zombie's next turn. 

Zap.  Ranged Spell Attack:  +4 to hit, range 60 ft., one target.  Hit:  9 (2d8) lightning damage, and the target must subtract 1d6 from the next attack roll it makes before the end of its next turn.  Miss:  The target takes half as much damage, and suffers no other effect.  

Discharge (Recharges after a short or long rest).  The zombie sends multiple arcs of lighting at up to six creatures within 30 ft. of itself.  Each target must make a DC 13 Dexterity saving throw, taking 16 (5d6) lightning damage on a failed save, or half as much damage on a successful one.  

Reactive Shock (3/day).  The zombie makes a Zap attack, ignoring any disadvantage.