Stone Camouflage. The giant has advantage on Dexterity (Stealth) checks made to hide in rocky terrain.

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.

Turn Resistance. The undead stone giant has advantage on saving throws against any effect that turns undead.

Multiattack. The giant makes two fist attacks or stone throw attacks.

Fist. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage, 10 (2d8) necrotic damage. The target must succeed on a DC 14 Constitution saving throw or its hit point maximum is reduced by an amount equal to the necrotic damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Stone Throw. Ranged Weapon Attack: +9 to hit, range 60/240 ft., one target. Hit: 28 (4d10 + 6) bludgeoning damage. If the target is a creature, it must succeed on a DC 17 Strength saving throw or be knocked prone.

Rock Catching. If a rock or similar object is hurled at the giant, the giant can, with a successful DC 10 Dexterity saving throw, catch the missile and take no bludgeoning damage from it.

Legendary Resistance (2/day). If the undead stone giant fails a saving throw, it can choose to succeed instead.

Magic Resistance. An undead stone giant has advantage on saving throws against spells and other magical effects.

An undead stone giant can take 3 legendary actions  choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The mountain giant regains spent legendary actions at the start of its turn.

• Grasping Soil. The undead stone giant commands the earth itself to assist in the fight. The giant chooses three creatures it can see within 60 feet. Each target must succeed on a DC 21 Dexterity saving throw or be restrained until the start of its next turn.
• Roar (Costs 2 Actions). The undead stone giant emits a tremendous growl. Each creature within 20 feet of the giant must make a DC 15 Constitution saving throw. On a failure, a creature takes 17 (5d6) thunder damage and is knocked probe. On a success, a creature takes half the damage and isn’t knocked prone.
• Spawn Zombie Ogre (Costs 3 Actions). A piece of the undead stone giant’s body tears away in the form of a zombie ogre. The zombie ogre acts on the same initiative count as the undead stone giant, obeying the undead stone giant’s commands and fighting until destroyed. The undead stone giant can have no more than five earth elementals under its control at one time.