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. On a success, the zombie drops to 1 hit point instead.

Multiattack. the zombie king makes two zombie kings hammer attacks and uses slam

Zombie kings hammer. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 7) bludgeoning damage.

Slam. the zombie king slams the ground and all creatures in a 15ft line must make a dc 16 dex save or are launched 20ft in the air the zombie king then leaps up and makes a zombie King's hammer attack against the closest target in the air and all fall damage is doubled to the target as they are slammed into the ground all other targets then fall to the ground prone.

Charge. the zombie king moves up to halve his speed and makes one attack with zombie kings hammer if the attack hits the target must make a dc 16 strength save or be knocked prone.