The Fallers are an alien race that are able to make almost-perfect copies of other species, after first absorbing them into their eggs. Faller eggs psionically lure, trap and absorb humans and other large animals, though humans are preferred. The absorbed life form is copied by the egg, the result is a being that looks like the original but is inhabited by the consciousness of a Faller. Fallers also eat members of the race they have copied for sustenance, not bothering to kill them first. Inside the Void, Fallers are distinguished by abnormally strong psychic abilities, while outside they are merely slightly stronger and more resistant to injury than the average for the species they have copied. The best known check for a Faller is a blood-draw; Faller blood is always bright blue, regardless of the copied race. The Faller brain is also distinct, but this can only be detected postmortem. Animals are always able to detect Fallers imitating their own species.
The Fallers are dropping eggs on the human-inhabited planet of Bienvenido, both due to a biological imperative to conquer planets and believing that the humans are vile animals who are denying the Fallers of their chance to reach The Heart. The eggs drop at a rate that strikes fear into the hearts of the humans, but not so great that the Fallers are seriously threatening to eliminate the humans, allowing human society to flourish, although a substantial portion of their society and economy is devoted to identifying and neutralizing a Fall, as they call a crop of eggs falling to ground. The rate is restricted due to the Faller "trees" being bunched together in a "Forest" over Benvenido, with only the outermost trees able to drop eggs. The temporal effects around the forest may also play a factor.
A key passage that explains the nature of the Fallers occurs in the first chapter that takes the viewpoint of Slvasta in The Abyss Beyond Dreams. When asked by Slvasta "Monster, why are you so evil?", a Faller who had Slvasta in her power replied "You make us; we are formed by you, your body, your mind. This - what I am, the way I think - it is inherited from your kind. It is vile. You, your species, is animal, brutal, despicable. Once we have exterminated you, it will take a generation to breed you out of us. But we will be free in the end." Slvasta replies "You will never defeat us. Freak monster. The Heart is for us, not you. You will never be fulfilled." The Faller replies "We have been before. We will be again".
Near the end of The Abyss Beyond Dreams, Nigel Sheldon manages to secure an egg and probe its nature, finding that the eggs are artificial constructs, rather than a stage in the life-cycle of a natural species. This is done by allowing the egg to eggsume and copy a serial killer (after mindwiping him; this choice was a moral compromise as neither Nigel nor Kysandra could justify subjecting anyone else to the process on purpose). After a thorough neural interrogation of the resulting duplicate, Nigel learns that the Fallers were a space-faring species native to the Milky Way that evolved into beings that could shape their forms at will, even becoming able to fly between the stars unaided at a considerable fraction of light speed. The Fallers colonized new planets by raining their eggs down, copying the local species, and supplanting them. But then one of their fleets was drawn into the Void. In their attempts to adapt their forms to accommodate the Void's basic drive to encourage species to fulfillment and absorb them into The Heart, some Fallers became Skylords, while the rest remained in the forest, trying to manipulate the fabric of the Void's space-time in order to escape.
Once freed from the Void, the Fallers are able to act more freely. Despite over 90% of the trees being destroyed in the blast when Bienvenido is freed from the Void, the egg drop rate drastically increases from the approximately 5000 remaining trees, which are now spread out in a large ring around the planet, raising the threat to the people below to one of extinction over the near (1000 year) term, although this is somewhat mitigated by the development of air interception, preventing many eggs from landing. Both while inside the Void and afterward, human society is mostly restricted to a single continent, with some raising concerns that the Fallers are unimpeded in their operations over the rest of the planet, but this is widely dismissed by both pre and post-transition governments, who view the actions of the Fallers as an attack on humanity and dismiss any possibility that they may be taking actions not directed toward that goal. In truth, without the ability to psychically detect Fallers, they have thoroughly infiltrated human society, operating nests throughout much of human territory, including inside both the criminal underworld and the very government created to fight them, with varying levels of effectiveness, while their forces outside the human continent grow unimpeded.
The most fearsome Fallers are Breeders, who are born from active Fallers rather than from Eggsumption. The first generation Fallers are able to specify the physical makeup of their offspring, combining aspects and producing customized combat Fallers and visibly indistinguishable, but physically superior Infiltrators. This is not foolproof, as occasionally mistakes, such as RoxWolf, a half-homiform, half-wolf, are produced; these mistakes are outcasts that are generally left to die but are tolerated if they can prove themselves useful.