Colossal

Innate Spellcasting. The colossal's innate spellcasting ability is Charisma (spell save DC 19). It can innately cast the following spells, requiring no material components:

At will: detect magic, feather fall, bless, light

3/day each: storm sphere, haste

2/day each: chain lightning, whirlwind

1/day each: foresight

Magic Weapons. The colossal's weapon attacks are magical.

Multiattack. The colossal makes two maul attacks, and one shield attack.

Maul. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 31 (6d6 + 10) bludgeoning damage.

Shield Bash. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the attack hits a creature, it must succeed on a DC 19 Constitution saving throw or be stunned until the colossal's next turn.

Voltage Ball (Recharge 5–6). The colossal hurls a magical 5 ft. sphere in a 500 ft. line. Each creature within 10 feet of the ball's path must make a DC 19 Dexterity saving throw, or take 11 (3d6) lightning damage, or half on a success. If the ball strikes a creature it stops. That creature must make a DC 19 Dexterity saving throw or take 42 (12d6) lightning damage, or half on a success. 

Fortify. The colossal's shield and maul meld into a two handed tower shield. The colossal gains a +5 bonus to AC, and is immune to line, cone, and ranged spells as long as it's facing the source. This transformation lasts until it's next turn.