TEXT: 
  Hell has its Tiamat. But the Abyss has its Beast.
  The Beast is terror-come-chaosflesh, a creature borne of
  fire and damnation. In its true form, The Beast appears as a
  seven-headed dragon some 60-feet tall. Its very flesh writhes
  with fire and the faces of the souls it consumes. Magma coats
  its scales, glistening like water off a duck's back.
  What few mad mortal dare worship the Beast know that
  were the Beast ever to escape the mortal realms, destruction
  would soon befall man. Only the bravest heroes would hope to
  challenge such a thing—even then, the creature is better left to
  its own devices.
  
  The Beast makes its lair in a pool of molten lava and burning
  flesh on one of the layers of the Abyss. It is rare that any
  creature—other fiends included—attempt to approach the
  Beast. The area considered the Lair of the Beast extends 5
  miles in all directions from the pool where the Beast
  hibernates.
  Lair Actions. On initiative count 20 (losing initiative ties)
  the Beast can take a lair action to cause one of the following
  effects.
  The Beast casts earthquake. Its spellcasting ability is
  Charisma (spell save DC 27), and it does not require
  material components. Once it uses this lair action it can't
  use it again for 7 days.
  A cyclone of wind and chaosfire forms surrounding the
  Beast, in a 120-foot radius cylinder that's one mile high
  centered on the Beast. All creatures within the cyclone
  when it appears automatically take 7 (2d6) fire damage
  plus 7 (2d6) necrotic damage. In addition, the creature
  must make a DC 27 Strength saving throw. On a failure, a
  target takes 28 (4d8 + 10) bludgeoning damage is flung up
  to 20 feet away from the Beast in a random direction and
  knocked prone. If a thrown target strikes an object, such as
  a wall or floor, the target takes 3 (1d6) bludgeoning
  damage every 10 feet it was thrown. If the target is thrown
  at another creature, that creature must succeed on a DC
  19 Dexterity saving throw or take the same damage and be
  knocked prone. If the saving throw is successful, the target
  takes half the bludgeoning damage and isn't flung away or
  knocked prone. The cyclone remains as long as the Beast
  concentrates on it, up to 1 minute, or until the Beast uses
  this lair action to create a new cyclone. On its turn, the
  Beast can use its action to move the cyclone up to 30 feet
  in a direction of its choice. The Beast is immune to the
  effects of its own cyclone.
  The Beast targets one creature that it can see within 120
  feet of it. The target must succeed on a DC 27 Dexterity
  saving throw or become enclosed in a floating sphere of
  acid. When the creature first enters the sphere and at the
  start of each of its turns it takes 14 (4d6) acid damage from
  the sphere. While inside the sphere, the creature is
  restrained. It can use its action on each of its turns to make
  a DC 27 Strength check to burst the sphere of acid, landing
  in an unoccupied space within 5 feet of the sphere.
  Otherwise, the sphere remains as long as The Beast
  concentrates on it (as if concentrating on a spell), up to 1
  minute. On its turn, the Beast can use its action to move
  the sphere up to 30 feet in a direction of its choice,
  including up or down.
  Regional Effects. The region containing the Beast's lair is
  warped by it magic, which creates one or more of the
  following effects:
  The area within 10 miles of the lair is unnaturally hot, with
  temperatures reaching as high as 200 degrees Fahrenheit.
  Rocks within 1 mile of the lair spontaneously burst into
  molten lava. Any creature within 10 feet of an exploding
  rock must make a DC 13 Dexterity saving, taking 10 (3d6)
  fire damage on a failed saving throw or half as much
  damage on a successful one.
  Thick, black clouds constantly linger in the sky, blotting out
  all natural light sources within 10 miles of the lair. Magical
  darkness spells and effect double their area.
  Patron Request by Darrion N.
  The First Demon Lord, the Beast, basically Satan,
  takes the form of either a black constantly
  shadowed humanoid figure who’s features you
  can’t make out or a seven headed dragon with
  burning crowns that emit and aura of flame to burn
  those nearby, the dragon form can conjure a
  massive ball of fire or several beams of fire, the ball
  basically being a super fireball, both forms can mark
  targets with Sin, dealing some psychic damage and
  making the target vulnerable to all damage for one
  turn like the Grave Cleric channel divinity.
  This is unofficial Fan Content permitted under the Fan Content Policy. Not approved/endorsed by Wizards. Portions of the materials used
  are property of Wizards of the Coast. ©Wizards of the Coast LLC.
  1
  
  This is unofficial Fan Content permitted under the Fan Content Policy. Not approved/endorsed by Wizards. Portions of the materials used
  are property of Wizards of the Coast. ©Wizards of the Coast LLC.
  
  Gargantuan fiend (demon), chaotic evil
  Armor Class 26 (natural armor)
  Hit Points 666 (36d20 + 288)
  Speed 50 ft., fly 90 ft.
  STR
  DEX
  CON
  INT
  WIS
  CHA
  30 (+10) 10 (+0) 27 (+8) 18 (+4) 27 (+8) 30 (+10)
  Saving Throws Int +17, Wis +17, Cha +19
  Skills Intimidation +19, Perception +17, Persuasion +19
  Damage Resistances cold, lightning
  Damage Immunities fire, necrotic, poison; bludgeoning,
  piercing, and slashing from nonmagical attacks
  Condition Immunities charmed, exhaustion, frightened,
  poisoned
  Senses truesight 120 ft., passive Perception 27
  Languages all, telepathy 120 ft.
  Challenge 30 (155,000 XP)
  Aura of Chaosfire. At the start of each of the Beast's
  turns, each creature within 10 feet of it takes 7 (2d6)
  fire damage plus 7 (2d6) necrotic damage and
  flammable objects in the aura that aren't being worn or
  carried ignite. A creature that touches the Beast or hits
  it with a melee attack while within 5 feet of it takes 7
  (2d6) fire damage plus 7 (2d6) necrotic damage.
  Legendary Resistance (7/Day). If the Beast fails a saving
  throw, it can choose to succeed instead.
  Magic Resistance. The Beast has advantage on saving
  throws against spells and magical effects.
  Regeneration. The Beast regains 10 hit points at the start
  of its turn. If it takes radiant damage, this trait doesn't
  function at the start of its next turn. The Beast dies only
  if it starts its turn with 0 hit points and doesn't
  regenerate.
  Actions
  Multiattack. The Beast can use its Gaze of Hopelessness.
  It then makes three attacks: two with its claws and one
  with its tail.
  Claw. Melee Weapon Attack: +19 to hit, reach 15 ft.,
  one target. Hit: 24 (4d6 + 10) slashing damage.
  Tail. Melee Weapon Attack: +19 to hit, reach 25 ft., one
  target. Hit:28 (4d8 + 10) piercing damage.
  Gaze of Hopelessness. The Beast targets one creature it
  can see within 90 feet of it that can see it. That creature
  must make a DC 27 Wisdom saving throw. A creature
  that is immune to being charmed has advantage on this
  saving throw. On a failed Wisdom saving throw, the
  creature is overcome by intense feelings of
  hopelessnesse. While overcome with this
  feeling, the target's movement is reduced by half, it has
  disadvantage on all attacks rolls, ability checks, and
  saving throws, and it can only take an action or a bonus
  action on its turn, not both. The feeling lasts for 1
  minute. At the end of each of the target's turns, it can
  repeat its saving throw, ending the effect on itself with a
  success. 
     If its initial saving throw is successful, the creature
  instead takes 7 (4d6) psychic damage, and if the
  creature has 100 hit points or less after taking this
  damage, it is stunned until the end of its next turn. If a
  creature succeeds on its saving throw or the effect ends
  for it, it is immune to the Beast's gaze for 24 hours.
  Shadow Form. The Beast magically polymorphs into a
  Medium humanoid shadow. It reverts to its true form if
  it dies. Any equipment it is wearing or carrying is
  absorbed or borne by the new form (the Beast's choice).
      In its shadow form, the Beast retains its alignment, hit
  points, Hit Dice, ability to speak, proficiencies, traits, lair
  actions, and Intelligence, Wisdom, and Charisma scores,
  as well as this action and its magic absorption reaction.
  Its statistics and capabilities are otherwise replaced by
  those of a shadow.
  Reactions
  Magic Absorption. Whenever the Beast is targeted by a
  spell of 6th-level or lower, it can use its reaction to
  make a Charisma saving throw against the spellcaster's
  save DC. On a successful saving throw, the spell fails
  and the Beast regains 1d10 hit points for each level of
  the spell cast upon it. If it fails its saving throw, the
  Beast does not absorb the spell and the spell goes off as
  intended, potentially with a second saving throw.
  Legendary Actions
  The Beast can take 7 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 Beast regains spent legendary
  actions at the start of its turn.
  Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one
  target. Hit: 26 (3d10 + 10) slashing damage plus 7
  (2d6) fire damage plus 7 (2d6) necrotic damage.
  Breathe Chaosfire (Costs 3 Actions). The Beast breathes a
  120-cone of chaosfire. Each creature in the area must
  succeed on a DC 25 Dexterity saving throw, taking 45
  (13d6) fire damage plus 45 (13d6) necrotic damage on
  a failed saving throw, or half as much damage on a
  successful one.
  Mark of the Beast (Costs 6 Actions). The Beast targets
  one creature it can see within 120 feet of it. The target
  must make a DC 27 Charisma saving throw. On a failed
  saving throw, the creature is cursed. While the creature
  is cursed, it becomes vulnerable to all damage as long as
  it remains cursed. Once the Beast uses this action, it
  can't use it again for 7 days.
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SOURCE: textdata/thevault/Dungeons & Dragons [multi]/5th Edition (5e)/3rd Party/DmDave/The Beast Patreon Request.pdf