Amphibious. The giant can breathe air and water.
Innate Spellcasting. The giant's innate spellcasting ability is Charisma (spell save DC 17). It can innately cast the following spells, requiring no material components:
At will: detect magic, feather fall, levitate, light
3/day each: control weather, water breathing
Greatsword. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 30 (6d6 + 9) slashing damage.
Lightning Strike. The giant hurls a magical lightning bolt at a point it can see within 500 feet of it. Each creature within 10 feet of that point must make a DC 17 Dexterity saving throw, taking 54 (12d8) lightning damage on a failed save, or half as much damage on a successful one.
Multiattack. The giant makes two greatsword attacks.
Rock. Ranged Weapon Attack: +14 to hit, range 60/240 ft., one target. Hit: 35 (4d12 + 9) bludgeoning damage.