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[All Patrons] Dungeons & Lairs #112 - Entrope Library (13th-Level Adventure)

Added 2025-04-16 18:38:11 +0000 UTC

Planar • Horror • Underwater • Underground • Aberration

GM Difficulty: Moderate
Player Difficulty: Moderate
Length: Moderate (1-2 Sessions)
Campaign Setting: Agnostic, Negative Energy Plane

This Fifth Edition adventure is designed to suit a party of three to five 13th-level characters. If played as part of the Thirteen Nails in the Mouth of God storyline, this adventure can be placed within Mortisphere as the characters explore the dead city, or it can be set as a stand-alone adventure in a setting of your choice.

Spellcasters will be useful here, especially those that can cast water breathing spells. The ability to detect magical glyphs will also be useful, as failure to miss the glyph in area 1b could make the combat with the entrope in area 2 much more difficult.

Characters who successfully complete this adventure should earn approximately one-third of the experienced needed to reach their next level.

Adventure Background

The Athanix Athenaeum was once a renowned planar library. Though believed lost centuries ago when a planar rift collapsed its upper halls and submerged it completely, rumors have resurfaced that a portion of the library’s lower levels still exists, intact and sealed beneath tons of ruin and pressure. Powerful wards and submerged architecture have preserved what lies below—along with whatever else may have crept in over the years.

The characters are tasked with recovering something from the library’s sunken lower levels. Whether it’s a specific text, artifact, or fragment of planar knowledge, their journey begins by navigating a flooded network of tunnels formed by the collapse. These jagged, waterlogged passages twist through old lecture halls, shattered archives, and coral-choked antechambers, eventually leading to a narrow route that descends into the untouched basement levels.

Though the objective lies below, danger is present throughout. Survivors of the collapse spoke of strange creatures drawn to the site—planar beasts twisted by the elemental bleed, waterlogged spirits clinging to lost knowledge, and worse things with too many mouths and too much hunger. The longer the library remains disturbed, the more unstable it becomes. Currents twist in strange directions, heat pockets warp perception, and whispers of forgotten languages fill the water like static.

Most who dive in never return. But those who do bring back wild tales—of fire flickering underwater, of corridors that lead to nowhere and yet somehow loop back, and of something massive moving deep in the dark. Whether truth or madness, one thing is certain: the Athanix Athenaeum is not as dead as it seems. And something in its depths is watching. Waiting. Changing.

Thirteen Nails in the Mouth of God. Deep within the Mortisphere lies the drowned husk of the Athanix Athenaeum, an ancient planar library long thought lost to time and ruin. Now partially fused with the Plane of Water and warped by elemental instability, the library exists in a flooded void where knowledge itself drowns.

The Remnants believe the Athenaeum holds one of the few surviving fragments of the Atlas Vex, a forbidden cartograph that may reveal the path to the Last Bastion—or at least a way out of this dying plane. But none among them have dared the descent. The region is unstable, swarming with planar anomalies and haunted by shifting elemental bleed. Worse, whispers speak of an entrope—one of the Corroded’s monstrous creations—gnawing away at the foundations of the library and unraveling the threads that keep the elements apart.

The characters are sent because they have little left to lose. If the Athenaeum holds even a piece of the truth, it could be the key to escaping the Negative Energy Plane.