Tytherion

A terrible realm of thirst, of darkness, of dragon-hunted deeps

Tytherion is a dominion in the Astral Sea plane in the World Axis cosmology, native to the Nentir Vale setting of Dungeons & Dragons. Called the Endless Night, it is the dark domain that Tiamat and Zehir share. No light can pierce its darkest depths, and both serpents and dragons haunt its otherworldly wilderness.

First touched upon in the 4e Manual of the Planes, it was further fleshed out in Secrets of the Astral Sea. It bears a passing resemblance to Gehenna and shares some of its features.

Planar Traits

• Type: Astral dominion.
• Size and Shape: Canyon maze about 100 miles across, 800 miles long, and up to 12 miles deep, surrounded by a belt of dry steppes 10 to 50 miles wide; bounded.
• Gravity: Normal.
• Mutability: Divinely mutable. (Deities control their own environs.)
• Alignment: Neutral Evil.
• Corruption: Attacks with the disease or poison keyword gain a +1 bonus to the attack roll. Healing powers restore only half as many hit points as normal.
• Necrotic Affinity: Attacks with the necrotic keyword gain a +1 bonus to the attack roll, and attacks with the radiant keyword deal half damage (ongoing radiant damage is not affected).

Geography

Tytherion's surface is something like Arizona or New Mexico, a desert of brushland, mesas, and rock formations riddled by canyons that criss-cross each other for hundreds of miles. These canyons may as well be bottomless pits since they go down for miles--we're talking the depth of the entire troposphere here. Were it not for the deities keeping things habitable, you'd be unable to breathe at the extreme altitudes due to the air pressure (or lack thereof.) Unlike Arizona, Tytherion has no sun, no moon, and no stars. The entire plane is completely dark at all times, except for the dim glow from lava rivers at the bottom of the canyons, and flashes of lighting from the occasional thunderstorm.

Zehir dominates the surface of Tytherion, while Tiamat rules the depths. Now, you may be wondering; why are the snakes up in the mountains and the dragons down underground? Well, originally, that is where they were, but both Tiamat and Zehir are greedy gods and each wanted to be the sole big cheese of Tytherion. So, one day, each of them got the bright idea to make a huge invasion of the other's territory... and something they did so at the exact same time, resulting in both of them sweeping cleanly through the now-vulnerable territory of their rival, only to realize that their former holdings had been captured at the same time. Whoops! And they've been stuck coping with it ever since, as they're too proud and miserly to just agree to swap things over, and too paranoid to risk trying another big invasion again.

Samaragd, the Serpent's Kingdom

The steppes, deserts, and higher canyons of Tytherion are the domain of Zehir.

Amun-Atl: This massive stepped pyramid of black stone stands atop an island plateau and is the seat of Zehir’s power. The god himself resides in a vast, lightless maze beneath the ziggurat, but he rarely emerges from his den to acknowledge the devotion of his worshipers. His yuan-ti exarch Atlathessk, the Seneschal of Night, runs the domain. Atlathessk demands a steady supply of living mortals to slay upon Zehir’s altars, which means that the denizens of Amun-Atl often deal with githyanki slavers or launch raids of their own against mortal worlds.

The Labyrinth: This labyrinth is the maze of tunnels in which Zehir dwells. Legend tells that only the most potent magic can create the slightest light within the Labyrinth. Portals within the labyrinth allow Zehir easy access to the Scales, or even to the Shadowfell and the mortal realm. The approximate locations of only two entrances are known: One lies beneath Amun-Atl, and the other on a cliff face overlooking the Obelisk of Night.

The Obelisk of Night: The Obelisk of Night was present when Zehir first drew Tytherion from the shadows of the Astral Sea and claimed it as his own. This towering monolith of smooth, ink-black stone rises from a crater near the border of Samaragd. Its surface is carved with winding symbols that vaguely resemble Supernal. These sigils resist all magical attempts at translation and seem, on occasion, to shift and waver. Mortals and exalted who study them for too long go blind. Rumor says that Zehir spends days at a stretch examining the Obelisk of Night from within his Labyrinth, perhaps struggling to translate the symbols or contemplating ancient secrets he has already interpreted. No one dares ask which it might be.

The Murder Pit: This gaping pit near the Obelisk of Night appears to be a great dark pool filled with viscous blackness. Said to be bottomless, the pit connects to none of Tytherion's other passages. According to legend and to Zehir's secret dogmas, by listening at the pit, those that use the proper meditative rituals can hear echoes—and occasionally see brief images—of any murder occurring anywhere in the cosmos.

Ennek-Vul: The largest of Samaragd's cliff-face temples, Ennek-Vul can be seen for miles. Its great entrance resembles the yawning maw of a serpent, and the stairs that rise toward it could accommodate a congregation of giants. Said to be the first temple constructed in Tytherion, Ennek-Vul fell out of use thousands of years ago when the Midnight Serpent offered it as a home to his greatest servant, whose name is commonly given as Merrshaulk.

The Ravenous Wood: A nasty forest of dead trees, dried brush, and desiccated brambles, so put your encounters with plant monsters here. Supposedly the Ravenous Wood hides the entrance to the lair of a terrible horror that predates or defies both Tiamat and Zehir.

Tabrol-Akla: Several of Zehir’s temples are built on the cliffs of the great chasms that plunge into Tytherion’s depths. Tabrol-Akla has the dubious distinction of being the lowest of these, hovering near the border of Azharul. Unsruuk, the exalted soul of an abnormally intelligent troglodyte, is the temple’s high priest and one of Zehir’s most potent darksworn. Similarly, the other priests who dwell in Tabrol-Akla and the warriors that serve them are among the most formidable of Samaragd, because Zehir’s exalted believe that if Tiamat’s forces ever move openly against Zehir, Tabrol-Akla will be one of their first targets.

• 
  Human sacrifices on a stepped pyramid in the middle of a lush jungle. How original.

Tiamat's Realm: Azharul

Tiamat rules in the lower canyons, haunting the volcanic depths of Tytherion.

Tiamat's Redoubt: The entrance to Tiamat’s lair is a great mountaintop fortress, with five looming watchtowers shaped like the heads and necks of sentinel dragons. The fortress is the gateway to the Caverns of Fiery Splendor, a broad entryway through which even the largest dragon can swoop, swiftly making a journey that would otherwise require hours of wending through the various catacombs. Guarded by Tiamat’s fiercest warriors, the fortress is nearly impregnable.

Caverns of Fiery Splendor: Beneath the deepest pit of Tytherion lies a vast cavern complex of flowing magma rivers and stone ramparts. This sprawling, labyrinthine cavern system is the lair of Tiamat, the Queen of Dragons. The darksworn spirits of evil dragonborn, outcast devils who have sworn allegiance to Tiamat, dark angels, and all manner of draconic spawn fly, crawl, and slither through these vast caves, fawning on the Dragon Queen. Five great wyrms—white, black, green, blue, and red—serve as Tiamat’s consorts and guard her hoard, a collection of treasure that beggars the imagination. Within Tiamat’s lair stand a set of adamantine gates over 100 feet tall. Tiamat does not reveal what might lie beyond them and has never allowed them to be opened.

Yithomel: Within this forbidding citadel, a cabal of Tiamat’s most powerful darksworn spirits rules over a vast region of Tytherion’s middle level canyons. These are the Seven Kings of Yith, mortal monarchs who each devoted their lives to a truly draconic lust for wealth and splendor. Each of the kings competes fiercely to amass more wealth and influence over mortal affairs than the others, with each of them engaged in an endless game of one-upmanship. Dozens of maruts who are bound by centuries-old contracts to protect their gluttonous employers guard the fortress.

• 
  When Tiamat's not in hell, she hangs out here.

Exalted of Tytherion

Mortal spirits who displeased Zehir in life are transformed into monsters. Those who gained his favor continue their service to the god of night in the afterlife as darksworn. They remain in their humanoid forms but are granted serpentine features—forked tongues, fangs, lidless eyes—as marks of Zehir’s favor (like yuan-ti malisons). Slain darksworn usually resurrect after a week or so, but souls sacrificed on Zehir's altars are permanently destroyed.

Tiamat doesn't seem to care much for her darksworn, leaving them to continue their greedy squabbles in the depths of the dominion. Many choose to leave, pursuing their vendettas or amassing wealth in other planar cities such as Sigil, Chernoggar, or even Hestavar.

Other Inhabitants

The archdevil Geryon was exiled to Tytherion after he lost control of Baator#Stygia, the fifth hell. Other outcast devils dwell here, including some loyalist spies. Probably Abishai too.

4e doesn't have any yugoloths but if it did, they would dwell here, since it's the closest thing to Gehenna in the World Axis cosmology.

The Crawling Castle: 4e's version of the Crawling City. It doesn't namedrop the General of Gehenna, instead stating the castle belongs to the cambion Vulkur Vaal, also known as Vaal the Flayer.

Hopelorn: The stronghold of the lich-lord Melif has also been moved to Tytherion.

The Scales: The astral archipelago of Tytherion is a dark and unpleasant place for exalted of Tiamat and Zehir to hide without leaving the plane. Supposedly a good place for paragon-level encounters, assuming you'd be epic level to brave the domain proper.