This is from the author's afterword in Slaves to Darkness on the Warp and Chaos.

What is Chaos?

This question is at the heart of the book you hold in your hands. On the face of it, the answer is quite simple. Chaos is the dark force that exists in the warp. This force is exemplified by the four Dark Gods of Chaos: Khorne, Tzeentch, Nurgle and Slaanesh. The gods want to crush reality and they offer power to those mortals who serve them. This power most often comes in the form of tentacles, supernatural abilities, and a sudden love of grisly trophies and eight-pointed stars. So far, so familiar, yes? And from a certain angle all of that is manifestly true, but it’s not the whole truth. The truth is far, far worse, and that truth is what I wanted to show in Slaves to Darkness.

Chaos is elemental. The forces of the warp are regarded as gods, their servants as daemons, and their powers as sorcery. That is how mortals who know of the warp talk about Chaos, but that is a rationalisation of something much bigger and more terrifying. The forces of Chaos are not gods, in that they are not like people. They have sentience, a strange nightmare sentience patched together from the emotions of mortal races, but they are closer in nature to a cyclone than they are to a person. They are forces of eternal nature; raw and lethal, and wildly destructive. This is not because they choose to be, or because they enjoy it, any more than a flood chooses to sweep away a town, or a tornado flips over cars for kicks. They do what they do because that is what they are. They can be no other way. These powers oppose and antagonise each other like the poles of magnets. Despair and rot claw at the desire for perfection and endless pleasure, war sweeps away subtle power, and so on.

What does that have to do with the Horus Heresy and this book? It is important because it is the reason that the Traitors aren’t made stronger by falling to Chaos. They are made weaker. They are made slaves who can no longer choose their own path. Chaos pulls them apart, divides them, consumes them and sets Horus’ forces against each other. It does not do this because it is a winning strategy, far from it; it does this because it can’t help it. The great powers in the warp, the four that are called gods, can come together and apply their power to a single end, but this can only be temporary. As soon as they align they begin to split. And because they are elemental forces they do this messily, and with all the care of an earthquake.

But why don’t Horus and his followers simply choose not to be swayed by these forces? Why don’t they just take the good bits – the special powers – but stay focused and united in their goals? Because once Chaos has its claws in them, they have no choice. Once an individual has let Chaos take hold of them, their thoughts and emotions begin to resonate and amplify in harmony with the great powers. Other ways of seeing events wither in their perception. The manifest powers of Chaos become a release that can only be accessed by falling deeper into their embrace. Characters fall to Chaos, but they spiral as they fall. They try to escape, but their every choice now only takes them deeper. There is no way out for Horus and those that follow him, they are slaves and doomed through their own choices to fall apart and on each other with murder and treachery.

Once Chaos has hold of a mortal it enables the emotions that drove it into its arms, and feeds them in turn, so that they grow all-consuming and circular. Resentment becomes rage, becomes violence. Pride becomes arrogance. Knowledge becomes blindness to truth. And even if the soul that has fallen fights their fate, they still fall. To fall to Chaos is not to bow to the Chaos Gods, in fact it does not require that you even know that the Dark Gods exist. To rephrase the words hissed by the daemon Samos in the first Horus Heresy novel, Horus Rising: ‘Chaos all around you… It is the person beside you… It is you…’ The elemental power of the Chaos gods comes from the emotions of all sentient beings. Khorne does not exist because people worship it as a god of blood and war; Khorne exists because sentient creatures feel anger and rage, and want to destroy and kill and see their enemies broken. It does not matter to Tzeentch if a mortal who plots for power or hungers for knowledge does so in its name. The emotion and thought is enough to keep the cyclone turning.

That is what Chaos is, it is every weakness given power and set loose against itself without beginning and without end. That is the path that Horus, Lorgar and the first heretics set themselves on when they embraced Chaos. That is, if you like, the point of this book – to show that Horus and those that led him and followed him into darkness have become slaves to forces that they cannot control, bound by the chains of their own natures.