Xilreth, the Lock of the Black Threshold

Xilreth takes the appearance of a beast made of spectral shards and swirling energy.

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 The battle against Xilreth has two phases:

• Phase 1: Four Seals float around Xilreth, each with AC 15 and 30 hit points. Xilreth is immune to all damage while the Seals remain intact. To make him vulnerable, players must break all four Seals. This vulnerability lasts for 3 rounds, after which the Seals regenerate.

• Phase 2: Triggered at 50% HP. everything in a 7-mile radius becomes a 'pocket' dimension. This area fragments and turns into many shards of existence that circle around Xilreth above a void that kills a player if they fall in. This effect goes away instantly after defeating Xilreth. Six Seals spawn instead of four. The Seals move to random positions at the end of each round, requiring players to chase and destroy them while Xilreth remains immune to damage.

In both phases, each seal emits a different musical note. The seals must be destroyed in order from lowest pitch to highest, otherwise they will not take damage. In phase two, players take 1d4 force damage if they break the seals in the wrong order.

Xilreth lair is a fragment of the Far Realms, a dimension of madness and chaos where the laws of physics and logic break down. The very fabric of reality here is unstable and alien, warped beyond recognition. The environment flickers and shifts, like a half-formed nightmare, with constantly changing hues and impossible geometries.

The ground is not solid but appears as a constantly shifting series of patches of starry void and surreal, organic landscapes. Towering, twisting spires of alien architecture, made from unknown materials, stretch upwards at strange angles, their surfaces pulsating and writhing. The air feels heavy, thick with an unspoken tension, as though the boundaries between this world and the next are perpetually on the verge of collapsing.

In the center of this chaos, Xilreth stands suspended, his form fractured into shifting, spectral pieces. Around him, the Seals float and rotate, held together by an unseen force, preventing the full scope of his power from being unleashed. Reality itself seems to bleed around these Seals, as though the space is holding its breath, waiting for the seals to be broken or for a new force to break through the cracks.

Lair Actions

On initiative count 20 (losing initiative ties), Xilreth takes a lair action to cause one of the following effects; Xilreth can’t use the same effect two rounds in a row:

• Reality Unravels:
  The space around Xilreth shatters, with entire sections of the environment tearing apart into nonsensical voids. All creatures within the lair must succeed on a DC 17 Wisdom saving throw or become disoriented, suffering disadvantage on their next attack roll or ability check.
• Psychic Distortion:
  A wave of mental chaos pulses through the area, warping the thoughts of all creatures. Each creature within the lair must succeed on a DC 17 Intelligence saving throw or take 10 (3d6) psychic damage and be confused (as per the confusion spell) until the start of Xilreth’s next turn.
• Seals Warp Reality:
  The Seals shift wildly across the battlefield, bending space around them. Each Seal teleports to a new random location. This movement causes space to warp, and creatures must succeed on a DC 17 Dexterity saving throw or take 14 (4d6) force damage as the very air around them compresses and shifts violently.

Regional Effects

The region containing Xilreth’s lair is influenced by the chaotic, mind-bending nature of the Far Realms, which creates one or more of the following effects, felt within a 1-mile radius of his lair:

• Unstable Reality:
  The laws of physics and nature become unpredictable. Weather patterns change without warning, shifting from calm skies to violent storms of psychic energy. Gravity seems to fluctuate, causing objects and creatures to become lighter or heavier at random. At the start of each day, there is a 20% chance that creatures within the area must make a DC 15 Wisdom saving throw or suffer disadvantage on all ability checks for the next 24 hours due to the warped space around them.
• Hallucinations of Madness:
  The minds of those within the radius of the lair are subjected to terrifying visions of alien horrors and cosmic truths that no mortal mind can comprehend. Each creature that starts its turn within 1 mile of the lair must succeed on a DC 15 Intelligence saving throw or be affected by the confusion spell for 1 minute. This effect can happen once per creature every 24 hours. The overwhelming nature of the hallucinations makes it difficult to focus on the task at hand.
• Mind Warp:
  The area is suffused with the raw psychic energy of the Far Realms. Creatures within the radius of Xilreth’s lair have their minds subtly affected by the madness that radiates from it. Every creature within the area has disadvantage on saving throws against being charmed or frightened. Additionally, when a creature within the area takes psychic damage, they must succeed on a DC 16 Constitution saving throw or suffer from the dazed condition until the end of their next turn.
• Absence of Light:
  The closer one moves to Xilreth’s lair, the less natural light exists. Daylight begins to fade the closer they get, and after 1 mile from the lair, creatures find themselves surrounded by an impenetrable darkness that resists magical light. This darkness is not a true darkness but a stretching of reality itself—light sources of 30 feet or less are dimmed to 10 feet, and magical light sources flicker and pulse unpredictably.

If the Xilreth dies, these effects fade over the course of 5d10 days.