[the following power assumes the abyss model is the 'black gash in reality' offering. not entirely a force of entropy, nor a nothingness, but rather a source of leakage between realities.]
the logrus is a power involving change; changing natures, with the occasional permanency for balance. whatever the forces of its creation, its forces were slow to shape the universe; the source of greatest reality was the shell it built around itself, and the grand fortress that evolved above it.
not far enough away, however, was an odd spot. a place resistant to the forces of the logrus, and not entirely connected to the shaping world. this place has been named the abyss, the pit, the void, and many other names referring to its bottomless and unusual nature.
the abyss can grow. things wander both in and out of it, but almost nothing of animal sentience willingly nears it. some of the creatures that have wandered out of it have bred with both animals and plants in shadows bordering it, providing them some resistance to it. sometimes the effect of the abyss extends and collapses a shadow, but this is very rare, and usually has more to do with primal chaos being loosed nearby. (primal chaos weakens the bonds of reality, allowing the abyss to devour what is left.)
many have died exploring it, certainly more than have returned in any wholeness. the abyss does not discriminate; it appears that the logrus is some small protection against its forces, but that shapeshifting is a significant detriment. endurance helps.
there is a race of beings that live within and beyond it. like tir-na nog'th in Amber, if you go far enough within, you can come out into a reality very different. (apply to your gm; chances are you will not survive the try.)
the abyss has three main veils. the first is a field which warps magic, up to and sometimes including the major powers. this is what spills out directly, being most damaging to Trump due to its great psyche interference. (in some campaigns, this may be enough to shatter a trump not reinforced by some other power.)
the second veil is a place of illusions and the formation of reality based on the creations of the mind. this place is a journey of the mind; the physical body is slowed here. many creatures wait here to be armored in the matter of their opponent's fears, the armor having the strength of the visitor's psyche. a good psyche can move faster, but makes anything that comes into existance stronger. this portion of the exploration is punctuated by sensory deprivation until one creates something solid. note also that other creatures can warp you as you warp them, which is only one reason shapeshifting is dangerous here. (this is where Brand generally dies. =) =))
the third veil is the monstrous setting of forces upon the character. raw energies that focus first upon magics, both item and internal, often destroying them (this is a side-effect of the roots of the universe.) then they seek out the psyche, sending blasts of distracting and destructive energies, in the forms of unknown mental sounds (echos of all of reality.)
past the veils are the creatures. most of them are incapable of interacting with the reality of the shadow-polarity universe, and thusly cannot rip open the abyss any farther to allow their entrance. creatures such as the unicorn and the serpent live here, moving their immense collections of life between the universes. (to someone 'attuning' themselves to the abyss, they can feel the vibrations of this movement.) [attuning; researching and otherwise getting familiar with it. not entirely safe.]
other, hungrier things are left to the gm's design and players' imagination.
however, to use this inbetween space, a character may be the result of a breeding of an extraordinary corpse or person who succeeded past all of the veils. i would recommend a high devotee cost. none of these powers will work in undershadow.
 points, relative invulnerability to abyssal forces, as well as detection of such forces. this includes the skill necessary to pass the first and second veils mostly unharmed. this is standard for all ty'iga, and halved in strength for other nearby beings. this includes a resistance against sorcery as there is a small amount of abyssal energy constantly being generated by the individual. this is the highest level of any abyssal power that allows the individual to have anyother major power of the Amber universe save shapeshifting.
 points, reality manipulation. with a psyche advantage, this can be used on the Real as well. by extending that abyssal energy, one can release some of the holds of Reality while they maintain the concentration. this is useful in shadow to turn shadow adversaries into mere ghosts; their weapons going through the character. this is also useful for shadow travel, as one weakens bonds between the worlds. after the concentration ends, things generally return to their original realities (the weaker the shadow, the longer it takes.) can be used on items, too. cannot target inspecifics, like weakening one's grasp of reality, or charisma. it does make one vulnerable to the powers of logrus and pattern, but this can be done in Amber and Chaos proper. proximity to the actual powers will make it more and more difficult to concentrate. trump contacts are affected; only an advanced trump artist can make a trump of someone with this ability, and contact with someone in this way is uncomfortable and all transmissions will be "blotchy" if the other individual is not familiar with abyssal energies.
 points, abyssal strengthening. includes the above, with an additional trump effect; one can use this now to block trump in their shadow (or, if at a shadownode, in the surrounding main shadows.) it takes this amount of power to weave abyssal blocks into pointbought planes. instead of sight, this power can be used to detect other powers by their scent. at this level, one can also remove the effects of the abyss by drawing the abyssal energies into oneself. this can clean up abyssal spills, if not draw the rift near Chaos closed (locking them forever in this universe? or until the next rift is opened, and perhaps in a worse place.) this also allows the abyssal master to eat magical energies, by dissolving them with the abyss; they can break down conjurations at one point every fortyfive minutes. this is refreshing to the individual involved, and in effect restores their endurance. at this level, the abyssal creature can survive for some small period of time without their bodies, more if they are in an area of the abyss.
 points, call the abyss. includes the above, as well as the ability to drive a hole in reality straight to the abyss, destroying shadow almost as if primal chaos. requires test against Real items, takes total concentration (nothing but latent defenses, except for psyche which is effectively barriered during this action except against another abyssal initiate.) this hole can be used for travel, as per the first and second veils. this is resisted by the whole of shadow, with it attempting to move around or collapse the tunnel. will often release nearby abyssal creatures into shadow from near the tunnel walls. will absorb passing, unprotected souls. can destroy Pattern ghosts. can be used to communicate with someone in the abyss. will cause total trump interference within any shadow the tunnel is held. will be easily detectable far and wide by creatures of abyssal nature. at this level, the abyssal creature can also use its knowledge of the nature of the universe to burrow down to the roots and tie something into reality matching Amber's, perhaps more, but not rip another hole in reality. can block lesser initiate's powers, but remove only those at the first level.
at seventyfive points i would venture that the abyssal initiate could begin to do things while holding open the pit, including call his kind through, or flooding Amber with abyssal energies (which the Pattern is resistant to, but not perfect against. Logrus is also resistant to abyssal energies, whereas primal chaos is useless against abyssal forces; perhaps at 90 points an abyssal creature could eat primal chaos.) i also imagine that at seventy five points, the person warps shadow wherever he travels, and has a definite psychic presence.
i am not a gm, so i do not know if these point values are fair, but i find them approximately what i would expect these powers to go for, if i were to be buying them. i believe the influence here is that creatures beyond the abyss are on a higher level in power than the average PC, but are limited in that the polarity of Shadow is difficult for them to interact with; their true strength is in the places inbetween, which is why i believe Trump may be related or opposing. [on the idea that when you cross through Trump, you're passing inbetween.]