Charybdis Tank

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

Magic Resistance. The tank has advantage on Saving Throws against Spells and other magical Effects.

Mode Change. When the tank is not in combat, its movement speed is doubled and it gains a 30 ft. burrowing speed.

Charge. If the tank moves at least 20 ft. straight toward a target and then hits it with a Ram Attack on the same turn, the target takes an extra 13 (2d12) bludgeoning damage. If the target is a creature, it must succeed on a DC 18 Strength saving throw or be pushed 10 feet away and knocked prone.

Multiattack. The tank makes three Harpoon or Machine Gun attacks, or one Ram attack and then two Harpoon or Machine Gun attacks.

Ram. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 23 (3d12 + 6) Bludgeoning damage. 

Harpoon. RangedWeapon Attack: +10 to hit, range 30/90 ft., one target. Hit: 15 (2d8 + 6) Piercing damage. The target must succeed on a DC 18 Strength saving throw or be pulled 20 feet in a straight line towards the tank.

Machine Gun. RangedWeapon Attack: +4 to hit, range 120/600 ft., one target. Hit: 9 (2d8) Piercing damage.

Artillery (Recharge 5-6). The tank uses one of the following artillery attacks.
Plasma Field. Each target within 20 feet of a point within 100 feet of the tank must make a DC 18 Dexterity saving throw, taking 39 (11d6) Lightning damage on a failed save, or half as much on a success. On a failed save, targets' movement speed is halved during their next turn.
Incendiary Shot. Each target in a 50-foot cone must make a DC 18 Dexterity saving throw, taking 45 (13d6) Fire damage on a failed save, or half as much on a success.
Doomsday Cannon. Each target within 20 feet of a point within 100 feet of the tank must make a DC 18 Constitution saving throw, taking (9d8) Necrotic damage on a failed save, or half as much on a success. On a failed save, targets are Blinded for one minute. At the end of each of its turns, a creature blinded by this ability can repeat the saving throw, ending the blindness on a success.

Charybdis Tank 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. Charybdis Tank regains spent legendary actions at the start of their turn.

Harpoon Attack. The tank makes a Harpoon attack.
Machine Gun Attack. The tank makes a Machine Gun Attack.
Cannon Shot (Costs 2 legendary actions). The tank fires its cannon. All targets within 10 feet of a point within 100 feet of the tank must make a DC 18 Dexterity saving throw, taking (8d6) Fire damage on a failed save or half as much on a success.