TEXT: Dark Souls Rules Variant
  Undead
  The undead in Dark Souls are quite unlike the undead of
  other fantasy roleplaying settings. While the undead cannot
  truly die, they gradually undergo a process called Hollowing
  as they die again and again, which ultimately robs them of
  their mind and spirit. Generally, an undead will continue
  being lucid and will not hollow completely while it still holds
  a deep purpose or mission.
  When you are reduced to 0 hit points, you die without
  making any death saving throws. You then return to a bonfire
  and become partially hollow. Your maximum hit points
  decrease by 10 each time you die, to a minimum of half your
  hit point maximum. Consuming a humanity restores your hit
  point maximum.
  Humanity
  A tiny black sprite found within living humans, humanity is an
  enigma to the residents of Dark Souls. It is certain, however,
  that all undead lack humanity, and find it fleeting. Consuming
  humanity reverses hollowing, but does not cure undeath; it
  merely postpones hollowing until the undead dies again.
  Powerful creatures, and sometimes the very lowliest, can be
  harvested for their humanity. It is said that humanity can be
  found in the deepest pit of the Abyss, but why it can be found
  in abundance there is a mystery.
  Bonfires
  The lands of Dark Souls are lined with singular points of
  heatless flame, each marked with a coiled sword and a heap
  of undead bones. These bonfires link the world and provide
  moments of respite for countless undead. When you take a
  short or long rest at a bonfire, you regain all expended hit
  points and refill all your estus flasks. Estus flasks and bonfire
  healing replace Hit Dice; you cannot rest apart from at a
  bonfire or recover hit points by expending hit dice.
  Estus Flasks
  The undead treasure these dull green flasks, which promise
  survival in a land dominated by death. Estus flasks provide
  swift healing, and are replenished at bonfires.
  You have a number of estus flasks equal to half your level,
  rounded up. You can drink an estus flask as a bonus action,
  which restores hit points equal to 1d6 + your Constitution
  modifier. All empty estus flasks restore when you rest at a
  bonfire.
  Lethality
  Because of the more abundant healing, and because Dark
  Souls is renowned for its considerable difficulty, all enemies
  deal twice as much damage. Especially dangerous enemies,
  like those which drop humanity upon their deaths, deal three
  times as much damage.
  DarkSouls3 - bosses splash by uger
  
SOURCE: textdata/thevault/Dungeons & Dragons [multi]/5th Edition (5e)/3rd Party/Mage Hand Press/Non-Published/Dark Souls.pdf