Ancient Tank

Immutable Form. The Tank is immune to any spell or effect that would alter its form.

Heavy Body. The Tank has a very heavy body. This gives the tank advantage against CON saving throws but disadvantage against DEX saving throws.

Magic Weapons. The Tank's weapons are magical.

3 Man Band. The 3 man crew gives the tank proficiency against INT and WIS saving throws. However, if a sleep spell engulfs the tank and does more than 72HP worth of sleep, the tank suffers from sleep. Additionally it cannot wake up from damage. On the start of each of its turns, the Tank rolls a d20. On a roll of 13 or higher, it wakes up.

Unfavorable. Some of the tank's abilities can put the tank into a special state called "Unfavorable". In this state, the tank's movement is reduced to 0ft. All its attacks have a -2 to hit and all attacks against it has advantage. Additionally all attacks score a critical hit on a roll of 18 or higher. This stacks with conditions that improve critical chance. The Tank must use an action to remove this state.

Search and Destroy. Some of the tank's abilities can put the tank into a special state called "Search and Destroy". In this state, tank's AC becomes 12 and it loses all its damage immunities and resistances. While in this state, the Patchwork Armor trait is removed. The tank must use a bonus action to remove this state.

Patchwork Armor. The mismatch of material that forms the chassis of the tanks has caused some serious armor vulnerabilities. If a critical hit is scored against the tank, then the total damage dealt from the critical attack is doubled.

Mystic Shell. The Iron shell protects the crew operating the tank from spells. The Tank is immune to most spells that causes status effects.

Ramming Speed. The Tank rushes forward at full speed toward a creature of your choice. All medium or lower sized characters in the way must make a DC 18 DEX saving throw or take 75(15d8 +7) bludgeoning damage. The target creature takes half damage on a successful save. When the tank uses this action, it moves 20ft ahead of the target creature. If a strong solid object is inside the area, the tank hits it and stops moving. It takes 45(10d8) damage and enters Unfavorable state. It cannot use this ability while it is in the Unfavorable state. If a large or huge creature targeted by this action or if it gets caught in the way, it can choose to instead make a DC27 Athletics(STR) check. On a failed check, the creature takes normal damage. On a successful check, the creature takes 27(5d8) damage and the Tank stops it's charge. The Tank can't move on it's next turn and it can't use reactions. It also can't use Ramming Speed. If the tank hits a Gargantuan creature, the Tank stops moving, takes 45(10d8) damage and goes in to the Unfavorable state. The creature must make a DC22 STR saving throw. On a failed save, the creature takes 120 damage and is pushed back by 10ft. On a success, the creature takes 60 damage and does not get pushed back.

Main Cannon. The tank fires a magically charged explosive from its main cannon. This attack cannot target anything within 10ft of the tank.
Ranged Weapon Attack: +5 to hit, Range: 100/300. Hit: 5(1d10) Bludgeoning damage.
Any creature within 5ft of the targeted point must make a DC13 DEX saving throw or take 3(1d6) piercing + 42(12d6) fire damage on a failed saving throw or half as much damage on a successful one. If the attack hits the target, no DEX saving throw is imposed on the target and the attack does its damage. If the attack misses, the target is to make a DEX save like everyone else. This ability has a 1 turn cool-down.

Mounted Gatling Gun. The tank fires its mounted Gatling gun. It can fire up to 3 times in one action.
Ranged Weapon Attack: +2 to hit, Range: 40/120. Hit: 10(3d6) piercing damage.

Manual Search.  This ability uses a bonus action. The tank enters the Search and Destroy state. The tank can make 3 Perception checks one after the other to find hidden opponents. If the hidden target or targets are found, the tank can use a free action to mark a single target that was hidden. The tank uses its reaction to fire its Main Cannon at the marked target with +3 to hit instead of the usual +5 at the start of the target's turn. The Main Cannon used this way ignores the cool-down for that ability. The tank cannot use Manual Search if it is in the Unfavorable state.