The Abyss
The Abyss is a flat featureless plain covered by eternal night (darkness 80%). The sky is rent by a huge crack, from which crimson light edges through. The land is filled with wandering bands of daemons, creatures of chaos with ITNs but no GTN, as they do not belong to any genera.
If more than seven people are within 1000 yards of each other in the Abyss, then daemons are summoned by the presence of life. Encountering another group of travellers in the Abyss can be extremely dangerous.
While you neither hunger nor thirst in the Abyss, regular meals help with energy and breaking up the monotonous journey. Sleep and rest are necessary, and of course dangerous.
Particular entities to avoid in the Abyss include the Lady of the Abyss, the Chaos Wind, Carnage, and Rampage.
The border between the plane of the Abyss and the Astral Plane of most worlds is a mile-high butte dropping into the Abyss.
The border between the Abyss and The Lands of Chaos is a steep slope, at the bottom of which is a wall, guarded by a garrison town, through which passes an archway covered in glyphs and runes. The road passes through this archway and up to House Kerberoth three-quarters of the way up the precipice.
There is a path leading up the cliff that is the border with Feralie if you know where to find it. The Lord of Metals has a mine/forge part way up the path.
Travel
Travel within the Abyss is by intent. It takes two weeks to reach the intended destination seemingly regardless of the speed of travel.
Celestial wings seem to be an exception to this, and will usually halve the travel time.
Some powerful entities have abilities or talents which allow them to travel faster.
Some methods of teleporting have been reported to allow crossing the Abyss in a day.
Rule of Seven
Groups of more than seven entities attract the attention of the denizens. The gathering on inquisitive Daemons make a bigger group which could attract more and things can quickly escalate to become very messy.
Some entities (ref Kerberites) have the ability to reduce the apparent number of travellers.
Denizens
Lady of the Abyss
The Lady of the Abyss is an immortal, she is cursed with insanity every other decade. In the 'sane' decades she is merely dangerous.
The scream of horses and clatter of carriage wheels is the only warning the Lady approaches, and advice is to flee at maximum pace, for it's not just devils who take the hindmost.
Chaos Wind
The Chaos Wind is a rainbow-streaked storm which shreds powerful locals and travellers alike.
Carnage
Carnage is a 30' wide red metal demon with wide spinning blades in front of a lobster-like body.
Razanil
Razanil is a 7'6" demon who runs fast, and doubles his mass every step he takes, resetting every 5 seconds; it is better to run up to him than be run over, but better yet never to have seen him at all.
Abyssal Daemons
Daemons are creatures of chaos, with mo moral dimension - some are good, many bad, some pleasant, many dangerous. They are not from Hell, nor Unholy (unlike evil devils from Hell). You can reduce or break them, but not kill them. Once broken, other daemons (or others?) may consume them to gain their power, through the talent of "Devouring". Daemons hold grudges, as once reduced, that power is never recovered, no matter how powerful the daemon later becomes.
Prince of Daemons
An immortal with power over Daemons who resides within the Abyss.