Multiattack

The giant attacks twice with its greatclub or twice with rocks.

Greatclub

Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage. If the target is a Large or smaller creature it makes a DC 19 Strength saving throw falling prone on a failure.

Rock

Ranged Weapon Attack: +9 to hit, range 60/240 ft., one target. Hit: 20 (4d6 + 6) bludgeoning damage. If the target is a Large or smaller creature it makes a DC 19 Strength saving throw. On a failure it is pushed 10 feet away from the giant and knocked prone. In lieu of a rock the giant can throw a grappled Medium or smaller creature up to 30 feet. On a hit the target and the thrown creature both take 15 (3d6 + 5) bludgeoning damage. On a miss only the thrown creature takes the damage. The thrown creature falls prone in an unoccupied space 5 feet from the target.

Stone Spikes

Magical spikes of stone explode from a point on a flat surface of unworked stone within 60 feet. Each creature within 10 feet of this point makes a Dexterity saving throw taking 28 (8d6) piercing damage on a failed save or half the damage on a success.

Avalanche (1/Day)

The stone giant magically creates an avalanche on a hill or mountainside centered on a point within 120 feet. Stones cascade down sloped or sheer stone surfaces within 60 feet of that point. Each non-stone giant creature within the affected area makes a Strength saving throw. On a failure a creature takes 17 (5d6) bludgeoning damage is knocked prone and moves with the avalanche until they reach a flat surface or the edge of the area. On a success the creature takes half damage.