Netherdeep

The Netherdeep is a strange realm, even in the infinte layers of the abyss. Filled with water, an egnimatic ruler in the heart of the netherdeep twisted the layer form to be a dangerous and enigmatic labyrinth full of traps and dead ends.
The large presence of Purple Isana, growing on every wall, every statue, ever crevasse of this layer is a good clue to the alien nature of the ruler identity, Xerathal.

Description

The Netherdeep is a lightless pit of water within the Abyss. Here, the water pressure is lethal to mortals who do not originate from the depth or the void. Purple Isana can be found all over the Netherdeep, and its haze contaminates the waters. What the Purple Isana corrupts, it grants the ability to withstand the overwhelming pressure.
The netherdeep is devided in three sections: the Grottoes of Regret, the Vents of Fury, and the Chasm of Yearning. These areas connect to each other and to a central node: the Heart of Despair. Above the three peripheral areas is seemingly infinite blackness; those attempting to swim upward never reach a surface, and they can only return to the area they left, rather than swim over the walls into a separate chamber. There are numerous secret doors concealing entrances to tunnels and hidden rooms. Purple Isana deposits provide what dim light exists in these parts of the plane, and all other light sources only illuminate to half the distance they ordinarily would.

Grottoes of Regret

The rough walls of Purple Isana-incrusted stone are interrupted by grand friezes of a large eye guiding crowds of people through the darkness. Ghosts of the beholder's past reenacting scenes it re-imagined, echoes of his past as a benevolent creature, linger before the carved walls. Crevasses that turn into winding tunnels connect the grottoes, which the beholder uses to navigate the layer.
The walls and floors in the Grottoes of Regret are festooned with ghostly corals of vibrant colors but of deadly touch, only interrupted by the glowing Purple Isana crystals.

Vents of Fury

The lowest chambers of the Netherdeep are known as the Vents of Fury due to the steam and thermal fissures in the floor. These vents are manifestations of the Beholder's wrath when he learned he was betrayed, and they also coalesce into abominous monsters that inhabit this region. These creatures are marred with ugly ridges and streaks of Purple Isana that resemble festering crabs.

Chasm of Yearning

This region of the Netherdeep reflects the furtive tunneling of a paranoid mind yearning to be at peace. In the beholder's mind, celestial and demons, mortals and immortals are pitted against him. In the Chasm of Yearning, not all is as it seems. It is a region filled with falsehoods and mirages of things too good to be true. If lit by magic, the walls of the region are a deep, soothing blue, except where interrupted by illusions or Purple Isana growths.
The beholder's paranoia is felt most strongly here, and the creatures that reside in this region of the Netherdeep appear as creepy stalkers and masters of illusions.

Heart of Despair

To enter the Heart of Despair, where Xerathal resides, a creature must have absorbed at least one Fragment of Xerathal's Mind - a tangible representation of the beholder's regret, paranoia, or wrath. Nine fragments exist in the Netherdeep. A fragment of Xerathal's mind in the netherdeep is a small Gazer.

Features

Ceilings and Floors

Tunnels have 10-foot-high ceilings, while a chamber or grotto has no ceiling unless its description states otherwise. If an area has no ceiling, assume that its walls are about 100 feet tall, and beyond them is a dark, underwater abyss. Creatures that swim upward never reach the surface and can only return downward to the area they left.
Unless otherwise noted, all tunnels have irregular floors that are at the same elevation as the floors of the chambers they adjoin.

Light

Locations in the Netherdeep are either engulfed in darkness or dimly lit by Purple Isana. All non-Isana light sources in the Netherdeep shed light half as far as usual. For example, a light spell gives off bright light in a 10-foot radius and dim light for an additional 10 feet.

Purple Isana

Purple Isana is plentiful throughout the Netherdeep. Brief contact with the purple mineral isn’t harmful, but contact lasting more than a few seconds can lead to purple Isana corruption. The save DC to resist the corruption is 20.

Secret Doors

Secret doors in the Netherdeep are made of the same Purple Isana-veined stone as the walls surrounding them. In any location that has a secret door, the text explains how the door can be found and opened.
A character who spends at least 10 minutes searching an area for secret doors can make a DC 20 Intelligence (Investigation) check at the end of that search. If the check succeeds and there’s a secret door to be found, the character finds it. Secret doors in the Netherdeep are unlocked unless otherwise noted, and they open easily despite their great weight.
When the characters pass through a secret door, it closes behind them unless it is held or wedged open. A character who has passed through a secret door once can always find and open it again from either side of the door.
A secret door has AC 17, 50 hit points, and immunity to poison and psychic damage. Any creature within 10 feet of a secret door that deals damage to it must make a DC 15 Constitution saving throw, taking 22 (4d10) psychic damage on a failed save, or half as much damage on a successful one.

Water Pressure

The Netherdeep is a supernatural abyss, and the water pressure it exerts on creatures within it is beyond anything most mortals can survive. Creatures, beside demons and abominations, not native to the Netherdeep take 35 (10d6) force damage at the end of every minute they spend within the Netherdeep, unless they are wearing or wielding an item infused with Purple Isana or have Purple Isana embedded in their bodies.

Hazard

Map

History

Xerathal was initially good-aligned beholder, with his eyes granting him truth-seeking abilities that could help create a just realm. He wished to aid mortals, and in his quest, angels and gods supported him. However, he soon realized he was merely a pawn in the celestial game.
This realization forced the natural paranoia of the beholder to surge back. If the angels were manipulating him, it was likely a scheme of the gods themselves. If the gods were involved, then everybody was against him. Quickly, he turned his back on the world.
Seeking refuge where the celestials couldn't reach him, he descended into the abyss, carving out a layer for himself. He created an underwater labyrinth representing his despair and disillusionment as he felt betrayed by mortals.
In his abyssal realm, demons tried to corrupt him, to use him for their own purposes, but their efforts only served to strengthen the beholder's paranoia. The more he interacted with the denizens of the Abyss, the more he saw betrayal and deception in every corner.
Xerathal is now the ruler of the Netherdeep.