TEXT: False Hydra
  The ideas and much of the flavor text of this homebrew came
  from http://goblinpunch.blogspot.com/2014/09/false-
  hydra.html.
  The False Hydra is a beast many don't believe exists. It is
  only discussed in the pale glow around a campfire or the
  candle light at a child's bedside, a story meant to dissuade
  lying and cheating. But it is very real, and very dangerous.
  Some say that they are grown from gods tears that have
  fallen to earth. Others say they are primordial and as ancient
  as the gods themselves. Still others believe they are the result
  of a mad wizards spell gone wrong. But it does not matter, for
  the effect is the same.
  Description
  The Hydra has pearl white skin and a large bulbous body
  with long spindly necks that twist up at impossible angles. Its
  heads almost resemble a horse but with vaguely human like
  fears with its white skin stretched tight against its deformed
  skull. It has neither eyes nor a nose but where its eyes should
  be lie two black holes, stretching into its skull. Its mouth
  extends away from the rest of its face with large weathered
  and cracked teeth, a long dry tongue flicks in and out. Its skin
  is covered in dirt and muck from when it burrows through the
  ground searching for its next prey.
  Song
  While the Hydra sings no one can directly interact or
  remember it, but that does not mean that it is completely
  immune to being detected. It can be seen in mirrors and
  while it can cut through fey ancestry quite easily those with
  fey lineage have an easier time breaking through its effects
  with magic. Feline animals can also see through the illusion
  quite easily.
  Even the general population is not completely immune to
  the song of the Hydra. While the Hydra may suppress the
  memories they are not erased, such as in modify memory.
  People may begin to feel a sense of anxiety when there is a
  disconnect between the message of the song and what they
  actually see. Eventually their brains will begin to revolt
  against the song and attempt to warn themselves of the
  danger they are in.
  These spells can see completely or partially through the
  Hydra's charm, but they don't stop the charm from working
  after they look away. Some may require a saving throw to
  work and others may work without one.
  Arcane Eye
  Detect Evil and Good
  Dispel Evil and Good
  Foresight
  Protection from Evil and Good
  See Invisibility
  True Seeing
  The Hydra must end its song to eat and so puts itself at
  risk, to minimize the risk it often attacks people who are
  alone and drags them underground to eat them without risk
  of being discovered. If the Hydra ever drops below half its HP
  or takes more than 1/10 its HP in one turn it ends its song
  and can not start it again until returns to above half its hit
  points. However, if someone forces the Hydra to stop singing
  they have advantage on resistancing the effect for for one
  round.
  Eating
  The hydar absorbs the basic memories of whatever it eats
  and as it grows
  Aging
  The more the Hydra consumes the stronger and larger it
  becomes, the more heads it grows. Depending on how much
  it eats and how long it has been alive a different stat block
  should be used, up to DM discussion. While a young hydra
  might be a walk in the park an adult hydra strikes fear into
  even the most seasoned adventures.
  Young False Hydra
  At this point the hydra has only recently hatched and begun
  to consume. It may only eat animals such as livestock and
  stray cats and dogs but it is still dangerous and will eat
  humanoids too if it gets the chance. It still only has one head
  and two arms that can emerge from the soil on the same
  spot, extending around 5ft up into the air. However, even if its
  body is small its hunger remains enormous.
  Adolescent False Hydra
  At this point in the hydra's life it is now consuming the
  general population of whatever town it is in. It may have
  between 2 and 4 heads that now can stretch out of the soil
  like eyes on a potato. It has grown more confident and
  reckless with its powers, consuming people that are more
  likely to cause problems if they are gone. Any adventures
  would start to notice strange occurrences throughout the
  local area
  Strange Occurrences
  1d6 Occurrence
  1 All but one candle is lit in the lampposts on the main
  street. No one can remember who lights them.
  2 The town beggars normal spot is vacant leaving a thin
  blanket and a had with a few copper pieces in it.
  3 Someone seems to run into something no one can
  see and then absentmindedly walks around it.
  4 Bells rang the day before at this time, why not now?
  5 Two fighting cats in a back alley way simultaneously
  start to hiss as something and then run away.
  6 A vendors stall or a store lies unattended, people
  avoid going into it.
  The general population would begin to feel a sense of
  unease as their minds desperately try to break from the
  hydra's charm. They may act emotionally when confronted
  with places and objects that were owned or frequented by
  their lost families, neibors, or friend.
  At this point the Hydra would most likely live in the sewers
  or naturally occurring tunnels beneath the town, it would also
  start to carve its own series of intricate, maze like tunnels
  and catacombs for its home. They would most likely have
  access points in sewers, cellars, or abandoned homes.
  
  Mature False Hydra
  By this stage the Hydra is nearly as large and terrifying as it
  will ever get, but still lacks a 7th head. By this point a sense of
  anxiety and paranoia should have swept the entire town as
  the populations brains revolt against the control, attempting
  to warn themselves of the danger.They find it painful to
  reminisce and so avoid it, their memories become deformed
  and twisted, identities become muddled as the cognitive
  dissonance and contradictory memories pile up. Friends
  faces seem subtly deformed and people experience huge
  swings in mood from one moment to the next. Role on the
  warnings table for ways a PC or NPC may try to warn
  themselves of their impending danger.
  Warnings
  1d6 Warning
  1 They wake up with a cryptic warning carved into their
  chest and skin under their fingernails.
  2 Reading what they wrote they find a sentence they
  don't remember writing begging them to leave town.
  3 They speak the name of people eaten by the hydra
  when referring to someone else.
  4 Their hand begins to act on its own at times,
  sometimes grabbing weapons and pointing
  vigorously.
  5 They have a dream of faceless people, and in the
  darkness a creature with dark holes for eyes.
  6 They start humming songs from their childhood but
  can't remember the lyrics.
  By this point the heads of the Hydra extend tens, if not
  hundreds of feet into the air, singing and swaying to an
  invisible breeze. The Hydra has grown so so many heads it no
  longer needs to end its song to eat and has almost free
  pickings of anyone it wants. It has also excavated large
  amounts of earth beneath the town and has also make a
  cavern deep beneath the earth where it will eventually grow
  into an adult.
  Adult False Hydra
  Before the Hydra enters its final stage of life it pulls all its
  heads underground and enters a sleep like state. For a short
  period its song ends and people may begin to remember who
  they have forgotten. During this time it is vulnerable to being
  remembered and attacked and so will make a lair
  underground where it can rest while doing so. Once this is
  done the Hydra grows its seventh head and broadcasts a
  permanant dominate person for a mile in every direction.
  PCs must make a DC 19 Wisdom saving throw or be under
  the hydra's control. If they suceed then the hydra will use its
  new minions to attempt to kill what it can not control.
  At this point the Hydra is still hungry but has become so
  engorged that it can no longer move on its own. It will
  command its mininions to dig it out from its subteranian lair,
  eathing as it goes and force them to slowly drag it to the next
  town. This will eventualy become unsistaniable however as it
  can neither force its minions to hunt for food (as this would
  let them leave ear shot) or continuously move its body (as it
  well eventualy become to large to move.
  Eventualy the Hydra will starve to death, wailing and singing
  as the last onzes of live leave its body
  
  Wail. When a creature that hits the Hydra with a
  melee attack is within 5ft. of the creature it takes
  1d4 psychic damage.
  Grapple. If the Hydra succeeds on a bite attack it
  can chose to grapple the creature instead of
  dealing damage if it is a size equal to or smaller
  than it.
  Song. If a creature is hostile to the Hydra when it
  begins its song the creature make a Wisdom saving
  throw DC 14. On a failure the creature drops out of
  combat, on a success the creature has
  disadvantage on all attacks against the hydra and
  must repeat the saving throw at the end of each of
  its turns if the song has not stopped.
  Actions
  Bite. Melee Weapon Attack: +5 to hit, reach 5ft.,
  one target. Hit 8 (1d6 + 5) bludgeoning damage
  Claw. Melee Weapon Attack: +5 to hit, reach 5ft.,
  one target. Hit 8 (1d10 + 3) slashing damage
  Song. The hydra can end or start its song as an
  action
  Reactions
  Shriek. Reaction to being hit with a melee attack:
  The Hydra sings its song giving it a +5 to AC
  Wail. When a creature that hits the Hydra with a
  melee attack is within 5ft. of the creature it takes
  2d4 psychic damage.
  Grapple. If the Hydra succeeds on a bite attack it
  can chose to grapple the creature instead of
  dealing damage if it is a size equal to or smaller
  than it.
  Song. If a creature is hostile to the Hydra when it
  begins its song the creature make a Wisdom saving
  throw DC 15. On a failure the creature drops out of
  combat, on a success the creature has
  disadvantage on all attacks against the hydra and
  must repeat the saving throw at the end of each of
  its turns if the song has not stopped.
  Actions
  Multiattack. The hydra can make two attacks with
  any combination of the Bite or Claw attacks
  Bite. Melee Weapon Attack: +5 to hit, reach
  10ft., one target. Hit 9 (2d6 + 6) bludgeoning
  damage
  Claw. Melee Weapon Attack: +5 to hit, reach
  10ft., one target. Hit 11 (1d10 + 6) slashing
  damage
  Song. The hydra can end or start its song as an
  action
  Young False Hydra
  Medium aberration, chaotic evil
  Armor Class 16
  Hit Points 50 (11d8)
  Speed Burrow 30ft.
  STR
  DEX
  CON
  INT
  WIS
  CHA
  7 (-2)
  16 (+3)
  11 (+0)
  9 (-1)
  10 (+0)
  17 (+4)
  Resistances psychic
  Condition Immunities charmed, blinded, prone
  Skills Perception +2
  Languages None
  Senses Tremorsense 30ft.
  Challenge 4
  Adolescent False Hydra
  Large aberration, chaotic evil
  Armor Class 14
  Hit Points 130 (26d10)
  Speed Burrow 30ft.
  STR
  DEX
  CON
  INT
  WIS
  CHA
  14 (+2) 12 (+1) 16 (+3) 11 (+0) 10 (+0) 19 (+4)
  Resistances psychic
  Condition Immunities charmed, blinded, prone
  Skills Perception +3
  Languages None
  Senses Tremorsense 60ft.
  Challenge 5
  
  Wail. When a creature that hits the Hydra with a
  melee attack is within 5ft. of the creature it takes
  3d4 psychic damage.
  Grapple. If the Hydra succeeds on a bite attack it
  can chose to grapple the creature instead of
  dealing damage if it is a size equal to or smaller
  than it.
  Song. If a creature is hostile to the Hydra when it
  begins its song the creature make a Wisdom saving
  throw DC 16. On a failure the creature drops out of
  combat, on a success the creature has
  disadvantage on all attacks against the hydra and
  must repeat the saving throw at the end of each of
  its turns if the song has not stopped.
  Actions
  Multiattack. The hydra can make three attacks with
  any combination of the Bite or Claw attacks
  Bite. Melee Weapon Attack: +6 to hit, reach 5ft.,
  one target. Hit 13 (3d6 + 4) bludgeoning damage
  Claw. Melee Weapon Attack: +6 to hit, reach 5ft.,
  one target. Hit 12 (2d8 + 4) slashing damage
  Song. The hydra can end or start its song as an
  action
  Reactions
  Shriek. Reaction to being hit with a melee attack:
  The Hydra sings its song giving it a +5 to AC
  Wail. When a creature that hits the Hydra with a
  melee attack is within 5ft. of the creature it takes
  3d4 psychic damage.
  Grapple. If the Hydra succeeds on a bite attack it
  can chose to grapple the creature instead of
  dealing damage if it is a size equal to or smaller
  than it.
  Song. If a creature is hostile to the Hydra when it
  begins its song the creature make a Wisdom saving
  throw DC 16. On a failure the creature drops out of
  combat, on a success the creature has
  disadvantage on all attacks against the hydra and
  must repeat the saving throw at the end of each of
  its turns if the song has not stopped.
  Actions
  Multiattack. The hydra can make three attacks with
  any combination of the Bite or Claw attacks
  Bite. Melee Weapon Attack: +6 to hit, reach 5ft.,
  one target. Hit 13 (3d6 + 4) bludgeoning damage
  Claw. Melee Weapon Attack: +6 to hit, reach 5ft.,
  one target. Hit 12 (2d8 + 4) slashing damage
  Song. The hydra can end or start its song as an
  action
  Mature False Hydra
  Huge aberration, chaotic evil
  Armor Class 15
  Hit Points 160 (24d12)
  Speed Heads/Arms Burrow 60ft.
  STR
  DEX
  CON
  INT
  WIS
  CHA
  16 (+3) 9 (-1) 18 (+4) 12 (+1) 11 (+0) 20 (+5)
  Resistances psychic
  Condition Immunities charmed, blinded, prone
  Skills Perception +3
  Languages Language of town it resides in
  Senses Tremorsense 120ft.
  Challenge 7
  Adult False Hydra
  Gargantuan aberration, chaotic evil
  Armor Class 17
  Hit Points 250 (24d12)
  Speed ---
  STR
  DEX
  CON
  INT
  WIS
  CHA
  16 (+3) 9 (-1) 18 (+4) 12 (+1) 11 (+0) 20 (+5)
  Resistances psychic
  Condition Immunities charmed, blinded, prone
  Skills Perception +3
  Languages Any language something it has eaten knows
  Senses Tremorsense 120ft.
  Challenge 12
  
SOURCE: textdata/thevault/Dungeons & Dragons [multi]/5th Edition (5e)/3rd Party/False Hydra.pdf