In the 1.3.6 update:

Demons:

• Added Soulseeker and Imitator. Reaper will be added later with the config update (probably next update).
• General:
  • Added "Battle Mode" mechanic to Necromancer, Skeleton King and Underworld Guardian.
• Dark Wolf:
  • Decreased minimum light level required for spawn from 9 to 7.
  • Now identified as Undead.
• Hanged Man:
  • Updated all animations.
  • Decreased movement speed from 0.45 to 0.42.
• Fallen King:
  • Added missing spawn restrictions.
• Skeleton King:
  • Changed step height to 3 blocks.
  • Decreased movement speed from 0.66 to 0.63.
  • Added chat messages.
  • Added "Call of the Skeleton King" ability.
  • Updated "Champion's Subordinates" ability: now spawns 8 skeletons instead of 9, improved procedure.
• Underworld Guardian:
  • Updated "Ground Shake" ability:
    • Minimal possible damage changed to 6.
    • Changed damage range to 25.
    • Now deals 6 more damage every 5 blocks closer to the Underworld Guardian (6 damage at 25 block range, 12 at 20, 18 at 15 etc.).
  • Changed step height to 2.4 blocks.
  • Disabled attack animation (Underworld Guardian now stomps the target instead of punching).
  • Now identified as Undead.
• Underworld Horror:
  • Changed step height to 1.6 blocks.

Blocks:

• Added Rotten Oak Door and Trapdoor.
• Blocks made of Rotten Oak can now be used as a fuel for furnaces.

Items:

• 1 new item: Strange Note.
• Midnight Necronomicon:
  • Added new information.
  • Added page numeration.
  • Fixed text mess in Resources, page 2.

Armory & Tools:

• Increased Demonic Armor enchantability from 9 to 17.
• Changed amount of Souls needed to create Soul-Cursed tools from 13 to 9.

Advancements:

• 4 new advancements:
  • Demonic Impudence (Let the Soulseeker steal a Dragon or Horrifying Soul from you).
  • The Fall of Humanity (Let the demons kill 666 human beings).
  • Demon Slayer (Kill 666 demons).
  • Full Moon Hunter (Kill 13 full moon demons during one full moon night).

Other:

• Updated welcoming message.
• Improved soul drop system. And some other minor changes and fixes, as usual.