Dungeon:

  1. Entrance: A large stone door blocks the entrance to the ruin. The door is inscribed with strange runes, and appears to be magically sealed.
    • The runes are written in Abysal. DC 12 Religion check: “Beware all who enter here, for the darkness within is great. Heed this warning, lest you become my prey.”
    • Inside is Alec, bleeding out.
  2. Foyer: The room is filled with dust and cobwebs, and a number of strange artifacts can be seen scattered around the room.
    • Filled with various wet and bloodied humanoid skeletons. DC 14 Medicine check will reveal that some skeletons are as old as several years whereas the newest ones are from a couple of days ago.
  3. Library: The walls of this room are lined with bookshelves and ancient tomes. The books are written in a language that none of the adventurers can understand.
    • A strange book is perched atop a pedestal in the center of the room. It radiates a faint magical aura and appears to have been sealed by a powerful enchantment. DC 15 Arcana check: The book is a collection of strange, eldritch rituals that the Half-Angel, Half-Demon has been using to attempt to restore its lost humanity.
  4. Hallway: A long hallway leads deeper into the ruin, and several doors can be seen along the walls.
    • The walls of the hallway are lined with strange symbols and ancient writing. DC 15 History check: The symbols and writing is of a long-forgotten cult devoted to a primordial being of chaos and darkness.
  5. Room of Traps: This room is filled with a variety of traps, ranging from darts to burning flames.
    • Various tools and materials can be found scattered around the room. DC 16 Investigation check: The tools and materials appear to have been used to construct the various traps.
  6. Boss Room: This room contains a large, magical construct that guards the entrance to the next room.
  7. Treasure Room: This room contains a large cache of treasure, including gold, gems, weapons, and magical items.

Outside the Entrance, crying on a tree is Arthur.

name: Half-Demon, Half-Angel
size: Medium
type: fiend
subtype: celestial
alignment: chaotic evil
ac: 14
hp: 39
hit_dice: 6d8 
speed: 30 ft., fly 60 ft.
stats: [18, 14, 16, 10, 13, 14]
saves:
  - strength: 8
  - dexterity: 5
  - constitution: 6
  - intelligence: 2
  - wisdom: 3
  - charisma: 4
skillsaves:
  - perception: 7
  - stealth: 5
senses: darkvision 60 ft., passive Perception 17
languages: Common, Abyssal 
damage_resistances: necrotic, radiant
damage_immunities: acid, cold, fire, lightning, thunder 
condition_immunities: charmed, frightened 
cr: 3
spells:
traits:
  - name: Magic Resistance
    desc: "The Half-Demon, Half-Angel has advantage on saving throws against spells and other magical effects."
  - name: Innate Spellcasting
    desc: "The Half-Demon, Half-Angel's spellcasting ability is Charisma (spell save DC 12, +4 to hit with spell attacks). It can innately cast the following spells, requiring no material components:
      - At will: darkness, fear
      - 1/day each: confusion, dominate person"
actions:
  - name: Multiattack
    desc: "The Half-Demon, Half-Angel can make three attacks: one with its bite and two with its claws."
  - name: Bite
    desc: "Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 9 (1d10 + 4) piercing damage."
  - name: Claw
    desc: "Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 4) slashing damage."
legendary_actions:
  - name: Detect
    desc: "The Half-Demon, Half-Angel makes a Wisdom (Perception) check."
  - name: Tail Attack
    desc: "The Half-Demon, Half-Angel makes a tail attack."
  - name: Wing Attack (Costs 2 Actions),
    desc: "The Half-Demon, Half-Angel beats its wings. Each creature within 15 ft. of the Half-Demon, Half-Angel must succeed on a DC 14 Dexterity saving throw or take 7 (2d6) bludgeoning damage and be knocked prone. The Half-Demon, Half-Angel can then fly up to half its flying speed."