The Grubs have the ability to absorb characteristics of aliens they encounter, by "eating" them and then utilising the biology of the alien they find to mutate themselves. They use this to communicate with other species without the use of translation technology.
A grub ship is known as a "void warren", and it is controlled by a central giant grub. The grub controls the actions of the ship, and the army of "helper" grubs (smaller versions of the giant grub). Both the helper grubs and the ship appear to be extensions of the one main grub, e.g. when a worker grub is hurt, the cry of pain comes from the main grub. Nothing in a void warren is wasted, and any grubs that are killed or injured are immediately dissolved and re-made into healthy new grubs or re-built into the ship itself. The grubs make use of a highly sophisticated force-field technology to help defend their ships.
In a flashback in Chasm City, Sky Haussman has an encounter with a grub. Another grub crash-landed on Yellowstone millions of years ago, and was still alive during the events of the novel Chasm City (though it was dead by the time of Redemption Ark). Examining the wreckage of this second grub's ship helped humans to eventually recreate its inertia-suppression technology. It was this grub that brought the Melding Plague to Yellowstone. The grubs appear to be immune to the plague, probably because of the plague-preventative agent that is produced by them. This liquid was concentrated and distilled into a form that was agreeable with the human metabolism, and was one way of protecting plague-susceptible implants and nanotech.
The Hamadryad is native to the Santiago peninsula on the planet of Sky's Edge. Most information about them is imparted in the novel Chasm City, where much of the action takes place on Sky's Edge. Hamadryads are named for the hamadryads of Greek mythology, for reasons which become obvious with an understanding of the animal's life cycle.
As the climate of Sky's Edge suffers no winter season, there was no ecological niche into which creatures analogous to mammals could evolve. Similarly, the vertebral column never evolved there, so all fauna are invertebrate poikilotherms, with the largest being broadly analogous to reptiles on Earth. Whilst the largest invertebrates on Earth are aquatic, such as squid, large invertebrates Sky's Edge also populate the land, possibly as the result of a catastrophic event having caused the oceans to shrink. The hamadryad is the largest land-based lifeform native to the planet.
The lack of a spine is handled, by evolution by animals maintaining structural rigidity "through the pressure of circulatory fluids alone, pumped by hundreds of hearts spread throughout the creature's volume".
The life cycle of the hamadryad is rather complex: juvenile and near-adult hamadryads are snake-like limbless creatures, of minimal intelligence, with a very simple central nervous system. The juvenile owned by Cahuella is described as being 12 metres in length and roughly as thick as an adult human's torso for most of that length, weighing more than a tonne. Being limbless, the creatures move like snakes on Earth. Their cold-bloodedness means that they generally move slowly, feeding infrequently, and are remarkably long-lived, with a longevity of hundreds of years. Being the largest native land-based lifeforms, the hamadryad has no real predators.
Juvenile and near-adult hamadryads have no apparent external eyes but, despite this, are capable of camouflaging themselves to match their surroundings. Contrary to this belief these pre-adult hamadryads do in fact have eyes. Eyes with remarkable visual acuity, set inside the upper roof of the jaw and spaced apart for binocular vision. Whilst hunting and preparing to ambush, prey are additionally triangulated with a host of other senses, such as infrared vision and smell.
At the end of the near-adult phase, adult hamadryads bond with a hamadryad tree, their bodies becoming part of the bark. The first explorers on Sky's Edge didn't investigate the hamadryad trees very well before the planetary war began, so the nature of the creatures' life cycle was not immediately apparent. The trees are relatively rare, but distributed across a large part of the Santiago peninsula, rising to a height of 40–50 metres above the forest floor. They are broadly cylindrical in shape, thickening towards the base, with an almost metallic-looking helical structure wrapped around the length of the trunk and a wide canopy at the top, tens of metres in diameter.
The first scientists to investigate these trees discovered unusual cell differentiation both radially and around the tree's perimeter — animal epithelial cells towards the outside, with soft, lipid membranes, and plant cells further inwards, with cellulose-like cell walls and chloroplasts.
The Inhibitors are the intelligence left over from a massive war — the Dawn War — that occurred between the first few civilizations that arose in the Milky Way galaxy. It is hinted in the books that they were once organic but that they later discarded their organic forms and became wholly machine. The hints of a quadrupedal, warm-blooded vertebrate (also known as mammalian) past can be faintly discerned in their architectures.
They are non-sapient machinery (referring to themselves as post-intelligent) functioning on unknown principles (speculated in the novel to be femtotechnology or "structured" spacetime) and capable of self-replication. Their technology often manifests as black cubes of "pure force" and is immune to conventional human weaponry; the machinery is easily capable of dodging most weapons thrown at it (usually temporary holes will appear and allow the shot to pass through) or is simply unaffected by it. The machinery can only be defeated by alien weapons supplied by the Hades Matrix or the Nestbuilders. They were created by the survivors of the Dawn War and their task is to inhibit the spread of intelligent life to individual planets or solar systems: the purpose, stated in Redemption Ark, being to shepherd the galaxy through a crisis 3 billion years (or 13 Galactic Turns) in the future: the collision of the Milky Way with the Andromeda Galaxy. By confining sapient life to only a few planets, they make the process of moving stars and systems (for collision avoidance during the crisis) far easier and more centralized, thus preserving life. However, when they have no choice, they tend to commit acts of xenocide in order to prevent life from spreading further.
They are not sapient; however, in order to supervise and control the process of xenocide, they are capable of forming a sapient overseer from many less-than-sapient machines. They also have some very advanced technology, and know about fifteen ways to kill a star, including one that allows the core material of a star to gush out and be used as a sort of solar flamethrower on planets (shown in Redemption Ark). This is the exception rather than the rule however, as being forced to destroy an entire system to cull a single species is viewed as a moral defeat. Normally it is much preferred to exercise (relative) restraint and preserve the long-term ability of the affected worlds to support life.
They do not actively monitor the galaxy in their wait for a new star faring culture to suppress, instead they plant a series of triggers near interesting phenomena or structures in the galaxy and wait for sapient life to activate those triggers. The Cerberus object around the neutron star Hades was one such object, and it was inadvertently activated by Dan Sylveste at the end of the book Revelation Space, thus triggering the events in the rest of the series.
They are called wolves by the Conjoiners, because they lurk in the blackness of interstellar space and attack in packs.
In the novels it becomes apparent that the Inhibitors are starting to fail in their mission as civilisations are getting further and further into space before being found and destroyed. In fact, in the last book (Absolution Gap), in the epilogue, it is shown that humanity, with technological assistance from other star faring, albeit hidden, cultures (for example, the Nestbuilders), were able to push back the Inhibitors and establish a space that was Inhibitor-free. However, this introduced the problem of Greenfly, a terraforming-replicator gone wrong that ravaged systems.
With the help of the Nestbuilders, humanity was able to overcome and push back the Inhibitors, and create a large area of Inhibitor-free space. It is not explained what they look like or what they are in Absolution Gap, but the story Galactic North describes them as insectoid beings that float using anti-gravity technology built into their bodies. They build structures out of diamond and possess weapons capable of cutting through spacetime.
One interesting thing about the Nestbuilders is that they are no longer sentient. However, for a long time a species called the Slugs has learned how to control the Nestbuilders' behaviour and subservient technology. The Slugs are small blue bags of protoplasm, seemingly too small and simple to house what humans grasp as sentience. They live in groups of up to a dozen between the gaps in the Nestbuilders' carapaces.
During the Greenfly crisis, it is not known what the actions of the Nestbuilders were, although it is known that they expressed doubt as to the effectiveness of even their weapons against the Greenfly.
The Pattern Jugglers are collectives of marine organisms that inhabit widely scattered water-covered planets. In Absolution Gap it was suggested that marine ecosystems continue to exist beneath the Pattern Jugglers. Whether or not they exhibit consciousness is up for debate, but in either case they are certainly advanced, capable of spontaneously gathering large rafts of complex organic systems and producing structures as advanced as superconductive strands. Human swimmers in Pattern Juggler seas appear to be infiltrated by the alien organisms(s), often resulting in out of body experiences and giving the swimmers short-lived periods of heightened consciousness and mental clarity. Some swimmers also report feelings that they encountered the memories or minds of other past swimmers during their immersion. Repeat swimmers, however, face the risk of the sea taking too much of a liking to them, in which case they never return to land. Reynolds' novella Turquoise Days goes into more detail about the Pattern Jugglers.
Pattern Jugglers record the neural structures of sentient beings who enter their oceans, and are capable of rewriting the brains of swimmers. Normally this involves giving them heightened abilities, such as expertise in mathematics. These abilities last only a short period of time, but sometimes they can last for the better part of a lifetime. They can sometimes be persuaded to record alien mind-forms onto humans.
It is likely that Mr. Reynolds borrowed the Pattern Jugglers name from a line in King Crimson's song "The Court of the Crimson King".
Most Pigs, however, are mute, lacking the ability to form words in either thought or speech. Some Pigs are more intelligent and can interact on a human level, but even they are not as intelligent as humans; Scorpio comments that he has little understanding of complex human interrelationships and when played music, only hears a series of sounds. All but the lowliest have enough intelligence to make them useful as servants, however. Such treatment, and their low standing in human society, leads many into crime. At the time of Chasm City, Pigs are only mentioned as either servants, or a criminal underclass. No Pig has reached a high position of authority among Humans except among the survivors of the planet Resurgam's destruction, where a large number of the rescue party were Pigs.
In Reynolds' The Prefect, the issue of racism between baseline humans and pigs is touched upon in greater and more personal depth than in any of Reynolds' previous novels in the Revelation Space Universe. Deputy Field Prefect Sparver Bancal is an augmented pig who, contrary to the apparent silence and low intelligence level possessed by pigs in previous novels is able to communicate and interact with humans and easily understands complex emotions and issues, such as his own discrimination and the impact of his having been promoted to the rank of Deputy (no pig ever having been ranked so highly before). He is also intelligent enough to work as a skilled software expert. At numerous junctions Bancal is called upon to either ignore or retaliate to actions of direct racism, but it is clear that the humans' attitude towards pigs is gradually changing in favour of forgiveness, compassion and acceptance.
Other members of the Revelation Space universe are unaware of the real nature of the Shrouders. In order to determine if Inhibitors were still active and that it was safe to leave the Shroud, one group of the Banished embedded a series of false messages into the Shroud that suggested that the Shrouders were a species that had gathered treasures and hidden them inside the Shroud. Lured by these messages, anyone who penetrated further into the Shroud would have semi-sentient programs and directives embedded by the Banished that would cause the intruders to trigger the Inhibitor device near the Banished home planet. If the Inhibitors responded to the triggering device then the Banished would know that the Inhibitors were still functioning. The program manifests itself in the novels as the "Sun Stealer" entity. It is capable of hacking virtually any human computer system and controlling starships. Eventually Ilia Volyova is able to defeat it with the Melding Plague, but not before it was able to activate the Inhibitors.
A second, dissenting group within the Banished planted a separate set of programs and directives to prevent the first group's plan from succeeding, believing that triggering the Inhibitors could lead to the discovery of the Banished hidden in the Shroud. It is implied that this agent manifests itself as the Mademoiselle, an entity with similar abilities which was able to penetrate even the Conjoiners' computer security systems.
There is some suggestion that the terraforming agent which the Shadows are fleeing from is the Greenfly; the agent they are fleeing has characteristics that correspond more or less perfectly with the Greenfly forces encountered at the end of Absolution Gap and Galactic North. If this is true, the Shadows could be descended from the human survivors at the end of Galactic North. Another possible explanation is that the either Greenfly or instructions for building them somehow came to our universe from the far brane.