Abyssal Core

The Guardian is Resistant to all damage types other than radiant. During certain actions, the Core of the Guardian is exposed. The AC of the core is 17 and is vulnerable to slashing, piercing, fire, necrotic, and radiant damage. All damage dealt to the Guardian that is not to its core can be regenerated. At the end of its turn, it regains 5d10 hit points minus however many hitpoints were lost from attacks on the Core.

Swat. Melee Unarmed Strike:  -5 to hit against flying targets. Reach 45 ft. One target. Hit: 45 (5d10+10)

Downward Swing. Melee Weapon Attack: Dexterity/Strength DC 16, reach 120 ft/15 ft circle, one target. Hit: 60 (6d10 + 15) Bludgeoning damage. Reveals Abyssal Core for 2 turns. Targeted creatures have advantage on the saving throw.

Sweeping Swing. Melee Weapon Attack: Dexterity/Strength DC 16, range 120ft/40 ft cone, only the last 15 ft of the cone. Any targets. Hit: 50 (5d12 + 10 ) Bludgeoning damage. Reveals Abyssal Core for 2 turns. 

God-Hand. Spellcasting: The Guardian raises its hand. A pitch-black sphere 20 ft in diameter forms at the end of the Guardian's hand. The sphere follows the hand of the Guardian and can be sustained for up to two turns.  Anything entering the sphere is instantly ripped apart and destroyed. If a creature touches the sphere it must make a Constitution saving throw. On success, the creature takes quarter damage, rounded up. On a failure, the creature takes 7d20 force damage. If the creature takes 40 or more damage past zero, it is disintegrated. The Guardian cannot take any other actions while God-Hand is active. God-Hand ends after two turns or when a creature is hit by it. This action has charges. The Guardian starts at zero charges and gains one charge every 15 rounds, up to one. When the Guardian gains a charge of God-Hand, the Abyssal Core is revealed until its next turn. When the Guardian uses God-Hand, the Abyssal Core is revealed for two rounds.

Rearrange. The Guardian can move its form to focus on a single strike. Parts of it can move to block incoming damage or remove creatures trying to climb it. This reaction can be used against attacks that cause any kind of damage the Guardian is not immune to. Against this attack, it gains immunity. This can be done once per 3 rounds. Using this reaction exposes the Abyssal Core for two turns unless the reaction is taken to protect the Abyssal Core. 

When the Guardian uses a legendary action its core is exposed. The core is covered two turns after the legendary action is used, regardless of if it uses more legendary actions.

Thundering Step (Costs 2 Actions). The Guardian puts a massive amount of force in a single step, and the ground in front of it shatters in a 40ft/40ft cone. All creatures in the cone must make a Dexterity saving throw with a DC of 22. If they fail, they must take 6d8 force damage and be knocked prone. If they succeed, the creature takes no damage but is knocked prone. If a creature is directly beneath the Guardian when it takes this action, the creature must succeed on a Dexterity saving throw with a DC of 20. If they fail, the creature is incapacitated and takes 20d8 bludgeoning damage. If they succeed, they take no damage but are stunned.

Fear Engine (Costs 2 Actions). The Guardian's core is revealed, and the powerful beating of the Abyssal Heart resonates through it. All creatures in a 60-foot wide and 200-foot long line in front of the Guardian must make a Wisdom saving throw. If they succeed they are deafened. If they fail, they take 4d8 thundering damage, are deafened, and are stunned until the end of the Guardian's turn. The Guardian can take an extra legendary action to extend the duration to an extra three turns. 

Sheer Will (Costs 1 action). If the Guardian fails any Strength, Constitution, or Wisdom throws, it can choose to succeed.