Legendary Resistance (5/Day). If the 7-Headed Beast of Revelation fails a saving throw, it can choose to succeed instead.

Mark of the Beast. When a creature with maximum amount of Mark of the Beast (3) receive damage from the 7-Headed Beast of Revelation: they receive the maximum damage of that attack or effect, afterward remove all Mark of the Beast from that creature. Mark of the Beast can be removed by Remove Curse spell and then that creature cannot have Mark of the Beast place on them for 1 minute, but if Remove Curse is cast at a higher level then increase the duration by an additional 30 seconds per higher level.

Massive Body. The 7-Headed Beast of Revelation cannot be grappled by creatures that are smaller size, and cannot be forcefully moved by non magical means. If the If 7-Headed Beast of Revelation makes a Claw or Horn attack, after the damage; the creature must make a DC 23 Strength or Dexterity Saving throw (the creature's choice), on fail the creature is pushed 10 ft, per their size difference.

Siege Monster. The 7-Headed Beast of Revelation deals double damage to objects and structures.

Trample. The 7-Headed Beast of Revelation can move through a creature with a smaller size. When it does the creature(s) must make a DC 23 Dexterity Saving throw, on fail; hit the creature with Claw but it deals Bludgeoning damage and make the failed creature prone. If the 7-Headed Beast of Revelation ends it's turn on top of a creature(s), push the creature(s) to be outside of 7-Headed Beast of Revelation's space of occupation.

Innate Spellcasting. The 7-Headed Beast of Revelations innate spellcasting ability is Charisma (spell save DC 23). It can innately cast the following spells requiring no material components. All spell are casted at 9th level.

3/day each: Call Lightning, Fireball, Lightning Bolt

2/day each: Circle of Death, Wall of Flame, Flame Strike

1/day each: Fire Storm, Finger of Death, Meteor Swarm

Multiattack. Make five attack.

Bite. Melee Weapon Attack: +15 to hit, reach 5 ft., 1 target. Hit: 22 (4d10 + 6) Piercing damage. 

Claw. Melee Weapon Attack: +15 to hit, reach 15 ft., 1 target. Hit: 18 (4d8 + 6) Slashing damage.

Horn. Melee Weapon Attack: +15 to hit, reach 10 ft., 1 target. Hit: 25 (4d12 + 6) Piercing damage.

Inferno Bolt. Ranger Spell Attack: +15 to hit, range (30/200) ft., 1 target. Hit: 15 (4d6 + 6) Fire damage.

Inferno Fire Breath (Recharge 5-6). The 7-Headed Beast of Revelation exhales hellfire in a 60-foot cone. Each creature in that area must make a DC 23 Dexterity saving throw, taking 24 (6d6) Fire damage, and 24 (6d6) Necrotic damage on a failed save, or half as much damage on a successful one.

Roar of Blasphemy (Recharge 6). Lets out a synchronized powerful roar from each head in a 150 ft. sphere with the 7-Headed Beast of Revelation as the center. Each creature in the area must make a 23 DC Constitution Saving throw, on a success half the damage. Deal 24 (6d6) Force damage, 24 (6d6) Thunder damage,  24 (6d6) Fire damage on a failed saved. After damage is dealt: each creature in the area also must make a 23 DC Wisdom Saving throw, on fail saved get maximum amount of Mark of the Beast.

Damning Roar (Recharge 4-6). Lets out a roar from each head in a 30 ft. sphere with the 7-Headed Beast of Revelations as the center. Each creature in the area must make a 23 DC Constitution Saving throw, on fail that creature gets one Mark of the Beast on them.

The 7-Headed Beast of Revelation 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. The 7-Headed Beast of Revelation regains spent legendary actions at the start of his turn.

Crowns of Damnation (Cost 1 Action). Target one creature the 7-Headed Beast of Revelation can see make a 23 DC Wisdom Saving throw, on fail one of the crown disappears from the one of the head of 7-Headed Beast of Revelation and appears on that creature's head. At the start of their turn it gains one Mark of the Beast, the creature can repeat the saving throw at the end of their turn. This effect is removed when the creature succeeds the saving throw, if the creature takes damage, or 7-Headed Beast of Revelation uses a Bonus Action to dismiss them. Then the crown disappears from the creature's head and appear back on the 7-Headed Beast of Revelation. This effect can affect up to 6 targets at once, but one target cannot have once than one crown on them at a time.

Attack (Costs 2 Actions). Make one attack.

Culling the Marked (Costs 3 Actions). The 7-Headed Beast of Revelation remove all Mark of the Beast from all creature(s) within 30 ft. and then gains 10 hit point per Mark of the Beast removed.