I was thinking the other day about the history of the 40k galaxy, the evolution of various xenos races within that galaxy, and how they all interact with the ruinous powers of the warp. I suspect that a lot of people won't agree with this, but it occurred to me that there virtually all xenos can be categorised into one of four broad classes according to their relationship with the warp and the chaos powers:
1. THE FALLEN
It is clear that over the multimillion year history of the Milky Way galaxy, a number of xenos races have fallen completely under the sway of the Ruinous Powers. Examples in the canon include the Saruthi, Laer and Yu'Vath. There are no doubt others, and there are some - like the Rak'Gol of the Koronus Expanse who are either completely under the influence of the warp, or so tainted that they are about to become a Fallen race.
Obviously the ways in which a race can become Fallen are various, but it seems clear that the influence of the warp acts like a sickness, a poison that spreads throughout a xenos culture, subsuming it completely. Races that are highly unified, or geographically limited to a single planet seem to be more prone to such intrusion, as the warped members of their own race are able to get hold of non-tainted holdouts and either convert or execute them.
So I'm suggesting that "Fallen" is a class of xenos race. In order to count as "Fallen" under my definition, every single member of the race must be under the influence of the Gods of Chaos. If there are any non-tainted holdouts who oppose the Chaos Powers, the race counts as "Tainted" instead - see below.
Fallen races are on a one way ticket to self destruction. The Saruthi in the Eisenhorn books are a good example. Their society becomes twisted, their original racial objectives and values totally swept away and they begin to rapidly wither and die as a species. The only thing that can prevent a Fallen race committing mass suicide or self immolation in a wave of atrocity is the presence of nearby xenos races of a different type who the Fallen can war with or infect with Chaos at the behest of the Dark Gods.
The Novel "Legion" shows that humanity almost became a Fallen race...at the peak of the Horus Heresy, if Horus had won, it would (potentially, arguably) have led to humanity turning inwards, engaging in brutal wars and wiping itself out within a century. This also suggests that there is a "tipping point" whereby if more than half of a race turns to chaos, there is a strong chance that the entire race is doomed to become Fallen.
2. THE TAINTED
The tainted is another class of xenos race. Tainted races have encountered chaos, and some (or at the very least, one) of the members of the race have turned to the worship of the Ruinous Powers. They are on the same path as the Fallen, but not so far advanced.
Under this definition, if even ONE member of the race has turned to chaos, then technically the race is Tainted.
The classic example of a Tainted race is humanity. At the height of the Horus Heresy, fully half of the human race was briefly under the control of the Chaos Gods: even now, ten thousand years later, a vast number of humans commit atrocities in the name of the Gods of Chaos on a daily basis.
Most xenos races are Tainted. The Orks are Tainted to a very minor degree - but if even one or two Stormboyz worship Khorne, they count as Tainted. The Eldar are Tainted too: the entire race, like humanity almost became Fallen, but stepped back from the precipice.
A race may spend its entire history being Tainted: they may never transcend to any other state. The Eldar are at least aware that there are other states out there, and aspire to them, but are unlikely to acheive them anytime soon.
Falling is not necessary the ultimate outcome of every Tainted race, but it is a logical possibility. If one individual from a race can Fall, then so can the race entire. Given the immortal patience of the ruinous powers, from their perspective once a xenos race achieves Tainted status, then it is only a matter of time before they Fall.
Most Tainted races will remain in a state of eternal conflict between those members of the race who have fallen to Chaos and those who haven't. This is a war for survival, and there can only be two outcomes: the race will either Fall or Ascend.
3. THE ASCENDED
The third class of xenos is really a theoretical one: there are no clear examples of them in the canon. However, logically, if a Fallen race is one where those individuals who worship chaos totally wipe out or convert those who oppose them, logically there is the theoretical possibility that the converse may take place: a Tainted race might completely wipe out all of those of their own kind who worship the Chaos powers.
An Ascended race has spent a period of time as a Tainted race, but has succeeded in wiping out its taint completely. However, the nature of Chaos is that it feeds off and is created the primal emotions and fears of living creatures. If, in wiping out their Tainted compatriots, a race despairs, seeks change, becomes angry or finds pleasure in its work, then that race opens itself to chaos reasserting itself at some future date.
Therefore, to truly Ascend, a race must not only defeat all parts of its own society that harbour warp energies, but defeat all aspects of their own soul which give rise to the Chaos powers. They must ascend to a higher state of enlightenment, becoming immune to those weaknesses of the soul which nurture the Gods of Chaos.
As I say, there are no examples in the canon of such a race. But in my putative scheme, Ascended xenos races have not only defeated chaos within their own society, but are intrinsically immune to chaos. They cannot fall to chaos because they do not generate negative warp energies. Every member of the race is a calm, enlightened Bodhisattva, and a deadly enemy to chaos.
There are examples of individuals achieving this state within the canon. The Sensei are said to be immune to the predations of chaos as they do no generate negative warp energies. (though this doesn't explain the "Grey Sensei.") Individual Illuminati may achieve something like this state, but only if they are immensely strong willed.
We don't know of any races ascending, though. It may be that the Old Ones achieved this state, though their disastrous relationship with Chaos implies otherwise. Some races aspire to Ascension: the Eldar path appears designed to move its exponents in this direction. The Eldar are akin to Buddhists who sincerely wish to walk the road to enlightenment, but are not always strong enough to do so. Perhaps only the greatest Farseers or the Harlequin Solitaires are able to call themselves Ascended. But perhaps not.
Any race which does Ascend is likely to be a very strange one by human standards. They would not necessarily act in a logical manner: self interest is not necessarily something which defines them. They would likely appear benign and mysterious, but as they know that Tainted races must find their own way to defeat Chaos, they are unlikely to actually act as mentors to them. They are likely to also be few in number: the act of wiping out the taint in their society may leave their numbers very low.
4. THE INNOCENT.
The Innocent are raceswhich have not fallen to chaos to any degree...yet.
This may be for any one of a number of reasons: sheer chance, cunning design, or ignorance of Chaos. They may be more intrinsically resistant to the wiles of chaos, but unless they actively Ascend, they are simply Innocent, and could become Tainted at any time.
The Tau are examples of an Innocent race. (Unless Commander Farsight's odd behaviour can be explained by the fact that he has fallen to Chaos, in which case, they are a Tainted race.) Farsight aside, there are no known examples of any member of the Tau race falling to chaos. It is suggested that they have less of a warp presence than other races, and as such may be less vulnerable to Falling. However it is unlikely that they are completely immune.
21st century humanity would count as an Innocent race: at this stage in human history, no one was aware of the warp and not a single human had fallen to it. (Unless you believe some of the Thousand Sons' theories about Magickal archaeology.)
The Tyranids are also arguably an innocent race. Their own warp presence appears to render them far less vulnerable to the influence of the warp...though Genestealers have been known to fall to Chaos, which arguably would render the whole of the Tyranids a Tainted rather than innocent race.
The Necrons are an Innocent race. As effectively dead creatures, they are not open to the soul pollution inherent in the living. The Necrontyr may have been innocent, but given their martial hubris may equally have been Tainted.
So the fate of most races would seem to be to go down the following path:
INNOCENT
I
I
FALLEN - TAINTED - ASCENDED
To move from Innocent to Tainted, and thereafter to become either Fallen or Ascended.
Anyway, that's one theoretical view of the interaction between xenos and chaos...any thoughts?

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