Crystal Giant

Crystaline Mythril. The Crystal Giant has crystalized mythril for it's entier body, with ordinary mythril armor and a shield meant purely for decoration. Some of its impressive exoskeleton can be seen through its powerful crystaline body.

Stalwart Defender. Only the original creator of the Crystal Giant can call of it's attacks. Otherwise, the Crystal Giant will continue to attack any perceived intruders. 

Multislash. The Crystal Giant make up to 4 attacks with its Crystal Greatsword.

Crystal Greatsword. Melee Weapon Attack: +10 to hit, reach 15 ft., 15 ft. arc. Hit:  44 (9d6 + 10) magic slashing damage. 

Crystal Lazer. Ranged Weapon Attack: +10 to hit, range 15/120 ft., 1 target. Hit: 46 (6d10 + 16) magic radiant/fire damage. Requires 3 round recharge before another use.

Multishot. The Crystal Giant fires off 3 weaker Crystal Lazer shots, dealing 24 (3d10 + 6) magic radiant/fire damage. Requires 3 round recharge before another use.

Heavy Shield Bash. The Crystal Giant charges forward 30 ft. with its shield (15ft. wide) in front of itself. All creatures in its path must make a DC 17 DEX saving throw. On a failed save, they take 55 (10d10 +5) bludgeoning damage and are knocked prone if still alive. On a successful save, they take half as much damage but aren't knocked prone. On a Natural 20, they successfully dodge out of the way, taking no damage.

Heavy Shield. The Crystal Giant holds its shield in front of itself, reducing damage taken by half until the next turn.

The Crystal Giant is a defender, but still understands that it must stay alive to fulfil its purpose. 

Quick Repair (Costs 2 Actions, 3/day). The Crystal Giant takes some time to do some maintenance, repairing damages dealt during the last round of combat so far. 

Ultimate Crystal Lazer (Costs 3 Actions, 1 use). The Crystal Giant uses up a ton of energy, letting loose a powerful 15 ft. by 15 ft. beam of energy that deals 85 (12d10 + 15) magic radiant/fire damage.

Self Destruct. The Crystal Giant generates a powerful blast, destroying itself and creating an explosion with a 30 ft. radius. All creatures within the blast must make a DC 19 DEX save or be caught up in the blast. On a failed save, they take 115 (12d10 + 35) fire damage. On a successful save, they take half as much damage. On a Natural 20, they dodge out of the way in time. On a Natural 1, they are consumed by the flames unless resistant to fire damage, in which case they escape with 1 HP left. All flamable objects left within the explosion become piles of ash.