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.

Undying Rage. On a success of its undead fortitude save, the colossus immediately makes an attack against one creature it can see within range of a punch attack.

Multiattack. The giant makes two attacks two with its greatclub or two with its punch.

Greatclub. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 18 (3d8 + 5) bludgeoning damage.

Punch: Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit 11 (3d6 + 5) bludgeoning damage

Rock. Ranged Weapon Attack: +8 to hit, range 60/240 ft., one target. Hit: 21 (3d10 + 5) bludgeoning damage.

Block. As a reaction the zombie colossus can use its arms to block a single melee attack that would hit and take half damage, it must be able to see the attack.

The zombie colossus has 3 legendary actions, which recharge at the beginning of each round. 

Earthquake (Costs 3 actions): Sacrificing its weapon the zombie collusus, smashes its weapon into the ground with enough force to great a minor earthquake breaking large sections of the ground around in a 30ft radius. Anyone caught in the radius must make a DC:16 Dexterity saving throw, or get caught up in the shifting rock taking 6d6 bludgeoning damage and falling prone. 

Additionally the terrain is then considered rough and makes it hard to walk on due to the fractures in the earth.

Rock (Costs 1 Action). Ranged Weapon Attack: +8 to hit, range 60/240 ft., one target. Hit: 21 (3d10 + 5) bludgeoning damage.

Multiattack (Costs 2 actions).  The giant makes two attacks two with its greatclub or two with its punch.