- "...mutable, ever-changing, yet always terrible to behold."
- — The Demonomicon of Iggwilv
Traits and environmentEdit
Hollow's Heart consists of a roughly circular continent approximately 9,000 miles in diameter. Beyond this, there is nothing but an endless waste of white sand. No sun is visible in the realm's empty black sky, but the entire surface is lit as if by the sun at noon. This eerily even light casts no shadows.
This layer's climate and terrain are diverse. Each region has its own unique geological features, weather, dangers, and range of inhabitants.
Locations and inhabitantsEdit
The Blood Dunes is the name of a vast red desert over a thousand miles across. The region is covered with a viscous red mixture of dried scabs and clotted blood. The temperature of this desert is severe.
A vast stretch of bloodsoaked mud here, known as the Red Rapture, is reserved as a execution ground for human captives. Large numbers of vrocks here serve as torturers and executioners. At the edge of this realm toward the ocean is the city of Karugoza (pop. 7,500), where captives are bought (for prices of around 100 gp) and processed.
The Demon's TeethEdit
A vast mountain range created personally by Fraz-Urb'luu in a years-long fit of rage. The edges of its peaks are are razor sharp, and four great lakes of bile lie in valleys between the mountains. Demon survivors of Fraz-Urb'luu's return live in these lakes, where they wage guerilla warfare against the demon lord.
The lowlands here are covered in the Deathroot Woods, a forest of Abyssal oak and pine trees whose roots resemble human bones. The woods are guarded by all manner of plant monsters and a circle of thirteen demon-blooded treants.
A long thin valley of white sand known as the Forever Gash cuts through the Demon's Teeth. For unknown reasons, it resists all efforts to allow itself to be re-shaped.
The Drooling JungleEdit
A massive forest of rubbery, murmuring trees, drooling poisonous ichor from their gaping mouths. The ground is a massive sheet of warm flesh, with black hair serving as the forest's undergrowth.
The Flensers is a menacing range of active volcanoes which line one edge of the Hollow sea. They constantly spew ash and lava into the air.
Between the Flensers and the sea lies the Harrowfen, a highly toxic swamp. These waters are poisonous to the touch, and as a result the area's only inhabitants are poison-immune creatures such as hezrou demons and skulvyn.
The Hollow SeaEdit
The shore of this vast ocean of foul, greenish water is lined with squalid settlements inhabited by savage humanoids and ruled by various demons. Orcs, goblins and ogres live here, where they serve as the realm's peasant militia, and human slaves are kept in some cities. The major settlements include Athawyn (pop. 9,000), Karantis (pop. 6,300), Bonepus (pop. 1,560), Magghat (pop. 3,200) and Uanthur (pop. 3000).
The massive iron temple Karantis overlooks the Hollow Sea. It is inhabited by 6,300 humanoid cultists of Fraz-Urb'luu. No humans are among their number; the Prince of Deception has not forgiven that race after his imprisonment by a human archmage.
A mundane forest inhabited by bands of prisoners escaped from the capital of Zoragmelok. Fraz-Urb'luu uses as his personal hunting grounds,
The capital of this plane is a city known as Zoragmelok. Here, the souls of cultists of Fraz-Urb'luu are held. Known as the Deceived, these humans sacrificed to the cult are held in a promised idyllic personal paradise, only for it to be revealed an illusion so that Fraz-Urb'luu can gloat over their despair.
Zoragmelok is sprawling and vast, protected by enormous adamantine walls. Human captives hang from these walls on hooks, kept alive by harpy clerics to prolong their torture. The city is guarded by powerful succubi.
Ugudenk, parasitic obyrith lord of the neighboring 177th layer of the Abyss, has tunnelled a connection into Hollow's Heart, where he has infested a small corner of that realm.Demonomicon (2010), p.79 Known as the Spiral of Ugudenk, it is one of the few pieces of Fraz-Urb'luu's domain that is not readily mutable by the will of Prince of Deception.
The bottomless Hollow Sea has a connection to the 177th layer.
Key to his power was a unique magical staff, with which he possessed the almost godlike ability to reshape his realm at will. This control over his home terrain made Fraz-Urb'luu virtually unassailable within his realm. 
Fraz-Urb'luu grew arrogant, and began taunting rival demon lords, including the tanar'ri princes Graz'zt and Demogorgon. Fraz-Urb'luu used his immense power to summon the demon lords to his realm, tauntin them before his court. This embarassment would not go unpunished.
Approximately 250 years ago, the Abyssal ruler of this layer, Fraz-Urb'luu, was defeated by Zagig Yragerne, the mad human archmage of the world of Oerth. Aided by rival demon lords, who provided Zagig with an artifact called the Ichor Lance and clues as to the truename of Fraz-Urb'luu, Zagig imprisoned Fraz-Urb'luu in a bas-relief beneath Castle Greyhawk for over 200 years.
The enemies of Fraz-Urb'luu—numerous and powerful— descended upon his realm in masse, slaughtering its helpless inhabitants and destroying his world. Unmaintained, the creation of Fraz-Urb'luu crumbled. Within a few years, nothing was left but a plain of white sand under a black sky.
Numerous minor demon lords from other layers of the Abyss took this opportunity to attempt a conquest of Hollow's Heart. The invaders each began to reshape captured territory in their own image, leaving the realm a fractured patchwork of clashing terrain.
The demon lord Baltazo captured the iron fortress of Shullen Gat on the first layer of the Abyss, a strategic position overlooking the 176th layer. However, the realm was highly contested by other demon lords and he was unable to make inroads.
Fraz-Urb'luu was eventually freed from his prison around 25 years ago, where he returned to what was left his ruined realm.
Exploding with rage, he quickly defeated the demon lords squabbling over his layer and recognized Baltazo with an official diplomatic role. Fraz-Urb'luu has yet to fully re-establish his dominance over the realm, and his old enemies Demogorgon and Graz'zt have made raids against him as punishment for humiliating them in the past.
Demogorgon himself at one point conquered half of Hollow's Heart, and could have captured the entire layer, but was forced to return to his own home realm to defend it from an open assault by Zuregurex, demon lord of the drowned dead. Demogorgon destroyed Zuregurex's realm in retaliation. Graz'zt and Socothbenoth also posed a significant threat, but were unable to oust him from his realm.
Fraz-Urb'luu's realm is first mentioned in the AD&D Planescape sourcebook Faces of Evil: The Fiends (1997), p.60.
The primary source on this realm is its entry in the D&D 3rd edition sourcebook Fiendish Codex I: Hordes of the Abyss (2006), p.142-144, expanded upon in Dragon #333 (Jul 2005) article Demonomicon of Iggwilv: Fraz-Urb'luu, Prince of Deception, pages 26-36. Hollow's Heart is briefly mentioned in Dragon #357's Demonomicon of Iggwilv: Demogorgon, Prince of Demons, pages 24-25; and Dragon #359's Demonomicon of Iggwilv: Apocrypha on page 52.
The plane was briefly mentioned in the D&D 4th edition Demonomicon (2010), p.79.
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 Demonomicon of Iggwilv: Fraz-Urb'luu, Prince of Deception, Dragon #333 (Jul 2005), p.26-36.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 Fiendish Codex I: Hordes of the Abyss (2006), p.142-144.
- ↑ 3.0 3.1 Fiendish Codex I: Hordes of the Abyss (2006), p.64-65.
- ↑ Demonomicon of Iggwilv: Apocrypha, Dragon #359 (Sep 2007), p.52.
- ↑ Faces of Evil: The Fiends (1997), p.60.
- ↑ Fiendish Codex I: Hordes of the Abyss (2006), p.155-157.
- ↑ According to Fiendish Codex I: Hordes of the Abyss (2006), he was freed 25 years ago, after being imprisoned for over 200 years. This means he was imprisoned for at least 225 years, and plausibly longer.
- ↑ Fiendish Codex I: Hordes of the Abyss (2006), p.115-117.
- ↑ Demonomicon of Iggwilv: Demogorgon, Prince of Demons, Dragon #357 (Jul 2007), p.24-25.