Effects of the Abyss, as compiled by Maeffly, sage of the Towers of the Sky, February 1167
A difficulty in creating a document about the specifics of the abyss is that the plane is one of Chaos, and there are no rules that can be depended upon for any large area. The main aspect of the Abyss which travelers there must keep in mind is that the various levels are often controlled by one or more powerful demons, often referred to as Powers, each of which has their own idea of what reality should be in their fiefs. The degree to which they control the environment they control is always unknown, but travelers will note that conditions can vary greatly from one area to the next. This can also affect more than just the environment – it can also affect perceptions, how magic works, and so on. In order to control their realms, most limit the ability of those in it with regards to flight, teleportation, and similar effects, usually blocking them entirely, unless their favor has been gained.
Another aspect of the Abyss is that many, if not most, of the Powers simply do not care who passes through their realms, as long as they do not stir up trouble. They will not in general choose to risk destruction of their followers for trivial passers-by. Thus, it may be possible to traverse great distances without being particularly concerned with the fact that you may well be mere miles from a demon prince or lord, or lesser but still very powerful demon ruler.
However, there is still much to be concerned with for the traveler in the Abyss. There may be barriers, guards, tolls, or groups of demons unaffiliated with a Power that will attack or threaten. Many spells function differently, either at a purely cosmetic level, through either being radically changed or completely useless in the Abyss. Some general ideas are known, and some specific limitations have been discerned. However, as with all things in the Chaos planes, these effects often cannot be specifically delineated, and can vary from place to place. There is the ever-present threat of the Soulforge raiders, who are always on the lookout for those they can enslave and sell to the Soulforgers, in the business of making obol steel, the coin of the realm. (While demons do trade in gold, magic, and the like, nothing is more valuable to them than souls.)
One particularly important note is that Wish, Limited Wish, Alter Reality and the like, if cast from a spell or device, are drawn from the will of some nearby power, and may not result in the desired effect. (Doing such will, it can be generally assumed, certainly arouse the attention of one or more of the nearby powers.) One exception to this would be if a wish had already been cast, but the effect was delayed, in sort of a contingency type effect; in such a case, the magic of the spell has already been granted, and should function normally.
Many skills will be reduced in efficacy in the Abyss – tracking, hiding in shadows, mapping, and so on, due to the unfamiliar nature of the environment, land and creatures. Specifics are simply impossible to determine.
The air, land, water and creatures of the Abyss tend to be infused with substances which affect those from the Prime as a weak poison, dealing a minor amount of damage each day. Poison immunity should negate this.
Animals from the Prime will not be very useable in the Abyss, as the effects of the plane on their senses and mind will essentially wreck their minds in hours or at most days. Unless one can procure local mounts, you should consign yourselves to traveling on foot.
Familiars from other planes will not be willing to travel to the Abyss, as their souls or spirits will be trapped there if they die, or if their master dies, the most horrible fate for such creatures.
You may see holes in the ground that open only into darkness. This may be conduits to higher or lower layers of the Abyss; they may, however, be the maw of enormous devouring creatures, or pits of destruction. They should be avoided. There are, much less commonly seen, portals to other planes; these are typically strongly guarded by those who claim them.
A few tomes reference something called ‘spell crystals’, which are apparently the physical manifestation of spells from outside the Abyss that were used to contact or subjugate some Abyssal creature. Generally these are dangerous to disturb, and I recommend not even approaching such things.
Magic notes in general –
The prime material plane, astral and ethereal, other outer planes and elemental planes cannot be accessed except from the very top layer of the Abyss, except in vary rare cases. Most Powers prevent access out of their planes, though they generally put up few if any barriers to entering their planes. Spells and objects like bag of holding, portable hole, and the like work normally. Some devices or spells specifically are linked to other planes, such as Zagyg’s Spell Component case, and so do not work.
Spells which summon or control specific creatures or types of creatures will not work, such as conjuring or summoning. Spells that target some general type, like ‘insects’, don’t apply to Abyssal insects; spells that might affect demons simply don’t work in the Abyss. It is possible that casting a summoning spell will get the attention of some local creature, but it will in no case compel it.
Cantrips do not work, at all. Casting such will merely toss that tiny bit of magic off into the plane, to be dispersed.
Most parts of the Abyss are very warm, or even very hot; this will reduce duration and efficacy of cold-related magic. Conversely, fire-related magic may be enhanced, but I have read only theories about this, not any actual experiences. I wouldn’t count on such.
All the news is not bad, however. Spells that create energy of a generic nature, such as fireball, flame strike, and so on, will work mostly normally, summoning similar energies from the plane where cast. These spells may evidence appearances, smells or sounds affected by the Abyss – a fireball may explode with a demonic scream, for instance – but actual function should, in general, be normal. Those spells which use true elemental energy, in whole or in part, whether it be from the four base elements or the paraelemental or quasielemental planes, will fail to function.
Magic notes in specific –
Abjurations do not work against the creatures that live here – protection from evil, and so on.
Alterations work normally most of the time, but may be ‘twisted’ in appearance, though not in meaningful properties. Lower level spells are more likely to have their output be altered than higher level spells.
Plane shift and the like can only be reliably achieved on the first layer of the Abyss. Below that, a caster can expect either that the spell will open to some other place, or that it will not work at all. In either case, probably a spell to be best avoided.
Creatures from this plane cannot be summoned against their will, though they may respond if they choose. They also cannot be coerced magically but might be negotiated with, if you appear to be dealing from a position of power. The weak are enslaved or killed.
Divinations are severely limited in the Abyss. The plane is so awash with chaotic evil that no other alignment can be detected. (Some sources state otherwise, but my researches indicate that no other alignments, even if specifically detected for, can be discerned.) Detect magic will detect the magic of the plane, making it impossible to determine if an item or location is enchanted; similarly, spells which detect the specific nature of magical items will fail. Commune will only be able to reach beings in the Abyss. True Sight and the like will expose the true nature of the Abyss to the viewer, paralyzing them for some minutes to more than an hour, and then possibly destroying their mind when the effect ends! Such beings who are not already Evil may be turned to that alignment against their will. I strongly urge you not risk such. Creatures with a very high wisdom who see things beyond normal will not be badly affected by this, though the memories may be troubling for a long time.
Enchantments/Charms – most creatures in the Abyss could be affected by Charm Monster, even if not a type you would normally consider as such; conversely, almost no creature in the abyss is affected by Charm Person or Hold Person. Confusion type spells are much more powerful in the Abyss, due to the nature of the realm.
Evocations/Invocations – most do not work, regarding contacting or reaching into other planes. Those which alter things nearby can be adversely affected.
Necromancy has no effect on beings native to the Abyss (unless the local Power has necromantic powers, in which case they may create undead, but outsiders still cannot).. The body of a creature from outside the abyss that dies there has a good chance that their body will be inhabited by a local creature, within a few days or weeks at most.
Magical Items in the Abyss
As in most outer planes, magic items that improve your combat, armor or resistance to harm will be reduced in power and may become effectively non-magical if not very strong. Oddly, this does not seem to affect potions or oils that mimic this sort of effect (oil of sharpness and the like), or non-permanent items like magical arrows, bolts, and so on. I suspect this is because the permanent enchantment of weapons, armor and the like bind their nature to the prime material plane, while the ephemeral nature of potions and arrows and such do not. [In game terms, all items with a plus lose one plus when crossing into the Astral, and another plus when going into any layer of the Abyss. Since you are going straight to the Abyss, your items will lose only one point.]
Regaining spells in the Abyss
Those casting clerical or druidical spells will only be able to regain level 1 and 2 spells, unless their deity resides in the Abyss, or they can somehow make a connection from their location – such would require some sort of very powerful planar magic, and if at all large, could attract the attention of the local Powers.
The turning of undead is harder in the Abyss; clerics and paladins will find it much more difficult to do so. However, a cleric whose deity resides in the Abyss counts will not only find it easier but will be able to affect many more creatures.
Addendum
There are other less-studied subjects of which I cannot provide any definite word, but about which you may extrapolate from the above.
Psionic abilities are probably similarly affected by being in the Abyss. Planar travel probably will work poorly, if at all. Many denizens of the Abyss are powerful psionicists, and as they are in their homes, their powers are generally stronger than those recorded in lore. Divination powers will probably give undependable results.
Items of great power may well work normally in the Abyss, if any such are possessed. However, unlike lesser items, they will be detectable and clearly seen as such, and will attract the attention of any denizens who see them. Bearers of such should be very careful about bringing such things to the Abyss, as the Powers there might well risk much to possess such things.
Related questions
Clerics or druids (if any of the latter exist, which I doubt) whose deity resides on the abyss gain spells normally. However, a power of the abyss might find it very interesting if a powerful cleric was on their plane, and they would detect this if spells higher than level two were prayed for. Spell effects for such characters are as described above, unless the priest in question has made some sort of arrangement with the local ruler.
The tuning fork for Plane Shift to travel to the abyss requires 5 g.p. weight of Obol Steel. When struck, it screams a disharmonic cacaphony of the voices of the creatures the fork was made from.
As far as possibly tainted demon gold, there are a limitless number of ways that it could be tainted without being magically cursed. I am not qualified to do testing on such, so I would suggest you take any samples you have to a magical metalsmith for more information.
I have heard one legend of a powerful magical instrument in the Abyss, Dagda's Harp. It was stolen from his grave many hundreds of years ago, ended up in the abyss, and has been traded around many times since. A local power might know where it is.
Food brought to the abyss will quickly absorb the unwholesomeness of the place, and will at minimum taste bad and look worse. Food prepared there may actually have baneful effects. Food created by cooking skills that have semi-magical benefits will be the same. Your best bet is probably to conjure food and choke it down as is, and not eat anything that has been exposed to the Abyss for long. Food sealed in an extra planar space will be fine until the device is opened, at which time it will be affected as if just brought in or created, even if the space is sealed back again.
Magical items or weapons which if cast away will return to their owner will function normally. I don't know why, as one would think the prohibition on flying and the like would prevent this.
Related notes on the effects of the Abyss, from Turin.
Attempts to leave or send contact outside the abyss will fail. When they find the demon portal to the enemy, they can step through into the astral and then get home via a portal the sword can make. He has a way to destroy the portal after they pass through. They will have to fight their way in, which will take days – they will get no result from tracking magic, so they’ll have to track clues, interrogate prisoners (not all of which are demons, many are evil souls), and so on. Turin will know it might be a few days, but he won’t know how long.
He says that certain special powers, granted by beings of great power, which might be 'once a day' or 'once a month', if used here, will be consumed and lost, as the connection to that being will be broken while in the Aybss and will not be restored, so use of those powers should be carefully considered.
Note that True Sight would be a very bad idea there.
The air is mildly poisonous, and each day at some point, different for each creature, they will take 1-2 points of damage and cough up blood. Poison immunity will prevent this.
Normal mounts will fare badly here from the above, also if they consume any local food or water, which will provide no sustenance and double the poison effect.
Familiars will NOT want to come to the abyss, if intelligent, unless C/E.
Neither teleportation, flying, or the like are possible here, even with physical wings.
There are no connections to the prime or elemental planes here, so no diamond dust can be conjured, among other things. In general, spells or items that create stuff won’t work. summoning will bring local equivalents (demonic mounts, conjured food will be bloody raw meat and ichor to drink). Such would also get the attention of local rulers, possibly.