BACK ON THREAD ' he original quote from lost and the damned and slaves to darkness was that not one i has fallen to the lure of chaos and if such was to heppen then they would be granted powers as great as those given to Horus.

so its not happened and the chances of it happening are as much as world peace i.e a million to one (though if you follow the terry prachett theory one in a millions heppen 1 in ten times) but the chance is there of course this is very old fluff and much has changed since i started playing the games

The way I see it the reason for all that effort being put into protecting them from falling to chaos (mindwiping, wards, careful screening etc.) must be that these things are needed to keep them pure.

So if a Grey Knight was being captured alive in a skirmish the Grey Knights lost and if he was being spirited away and maybe kept in a coma while his hexagrammic wards were being removed and systematically being replaced by devotional symbols of chaos, and he was being brought into contact with corrupted substances (and maybe being fed with them and have them inserted into his bloodstream) then I do see a chance for a fallen Grey Knight there.

 I've heard it said that Grey Knights are immune to corruption because the hexagrammatic wards in their skin and bones sets them on fire and kills them if they ever get corrupted in the slightest. It's less total impunity and more of a last-ditch failsafe. So a traitor Grey Knight not only doesn't make sense, it's also impossible.

On the other hand, I did have an idea about a Chaos counterpart for the Grey Knights; unlike regular Knights, they'd be psychic children of Chaos Cultists raised from a very young age into Imperium-hating zealots, equipped with Terminator armor and the most powerful, daemon-infested weapons the Dark Mechanicus could provide. In-character, the person who came up with this idea was a Slaanesh worshiper, and basically came up with the "Black Knights" as the ultimate blasphemy; a giant middle-finger to the Grey Knights and everything they stood for.

The "Black Knight" concept sounds cool, and it would be interesting to see how elite you could get.

I formerly thought a Chaos Grey Knight was impossible. For one thing, they can't gain Corruption points. For another, there's the possible self destruct method, though don't tell me it's completely impossible to remove, I don't buy that.

But after noticing the passage in Black Crusade about how a Grey Knight to turn to Chaos would be granted as much favor and power as Horus himself, you friggin can't tell me that the person who wrote it isn't giving a nod of approval to the concept! Unfortunately, I can't, for the life of me, figure out how to run such a concept and not have it be lame…

…Grey Knights barely seem to have any personality that could have a reason to seek after Chaos to begin with, their sole purpose is to destroy Chaos. Like, there's no "oh noes my girlfriend/planet/Emperor is going to be DEEEEEEAAAAAAAAAAD, I can only save them by going to KAYOSS!" for them, they don't give a damn, as far as I can tell. There's even a Grey Knight who already "knows" the Imperium is going to fall to Chaos and that will not stop him.

I rather liked Horus's reason (he saw what the Imperium would become). That doesn't work, though, if you already know the Imperium is crappy.

 GK simply cannot fall to chaos, they are so thoroughly trained, equipped, prepared and contempuous of chaos that it'd require direct intervetion of a chaos god themselves to have almost any effect on one, and that singular GK has his whole battle force backing him up.
Renegeade Ordo Malleus would make a much better plot device.

Its less accurate to say Grey Knights can't fall to Chaos and more accurate to say they haven't fallen to Chaos and that it is unlikely for them to do so, because you simply don't know. At least one has come very, very, VERY close to falling to Khorne (we can call him Sindri Jr.) upon having a Collar of Khorne placed on him. Remember, even the Emperor himself is inevitably losing the battle against Chaos -- the only question is whether his spirit will break first, OR HIS BODY!

"Even the Golden Throne cannot keep the cells of the Emperor's dead body alive forever. Over the millenia the link between his soul and body has become increasingly tenuous. Worse of all, the Powers of Chaos have begun to infiltrate his mind, sowing seeds of doubt, dissolution and fear. It is impossible to say for how long the Emperor can survive in this condition. It is unlikely that even he really knows how much time is left to him before the tenuous hold upon his physical body is broken by weakness or finally rent apart by insanity." Lost and Damned, p185.

I don't think its a storyline I'd ever run (I'm not sure Grey Knights as PCs even makes sense -- and I'm a person who thinks Chaos Spawn would make great PCs), but Black Crusade does give a nod of approval to the concept with the line of a fallen Grey Knight becoming the Chosen One of Chaos, and I doubt that Grey Knights are truly each on par with the Emperor himself (who is, himself, not immune to Chaos), but who knows. Then again, if they really do take a million psyker casualties each to create, then yeah, they had better be on the level of the Emperor.

To be honest, I'd argue that a resistance to the influence of Chaos is less a matter of raw strength of will and more a matter of psychology. Willpower helps keep Chaos at bay in the face of temptation.

Consider - Orks cannot typically be influenced by Chaos, not because they're innately strong-willed (indeed, Orks have extremely poor impulse control), but because the Chaos Gods have very little that can tempt an Ork. Being creatures of few desires, all of them easily-fulfilled in the short term, there is little that Chaos can offer that Orks can see a need for, under most circumstances.

I see Grey Knights in a similar light - avenues of corruptions have been specifically engineered out of individuals who are already strong-willed and psychologically resilient. Mind-scrubbing and reprogramming, intense hypno-indoctrination, application of wards through internal scrimshaw, scarification and tattooing. The means by which Chaos can gain entry are deliberately and forcibly closed. Physical warding to limit dangerous exposure, and psychological resculpting and personality atrophy to limit the freedom of choice necessary to embrace Chaos makes a Grey Knight exceptionally resistant to Chaos, though artificially so. By comparison, the Primarchs (a central example of powerful beings subverted by Chaos) were of exceptional, superhuman willpower… but Chaos entered them through their psychology, taking advantage of personality flaws, desires and fears.

Chaos does not batter against a target's defences, but tricks them into opening the door. A Grey Knight's psychological doors have been locked and barred by someone else, and the key has been thrown away.

That's a pretty apt way of describing it. They lack the usual personality traits that permit them to be corrupted.  

Still, there's probably some scenario in which it'd be possible for a Grey Knight to regrow such personality traits. Probably never fully, but complete stasis between the extremes of Nurgle and Tzeentch, for instance, is probably a fragile thing for a sentient being. They are kept within, afaik, a constant psychic gestalt of their comrades and in an extremely controlled environment at all times. This implies that they are in such a controlled environment for a reason.

Going along with this theory, the one time a Grey Knight came close to falling, he had his access to psychic powers cut off by a collar of Khorne and was forced to fight gladiator games.  It didn't go all the way, but who knows what would have happened if he wasn't rescued. A similar situation over months? Years? Decades? Centuries? It may very well do him in.

Orks, notably, have the same exact corruption immunity GKs have, but they've still managed to be driven to chaos before.

Again, I'm not suggesting I'd do it or that it'd be a terribly good storyline, but at the very least, its possible for Grey Knights to be literally driven insane by Chaos (which is not remotely easy. The question then becomes, what would a completely insane Grey Knight be like? Would a completely insane Grey Knight still be able to keep up his mental protection?

I'm PROBABLY going to say, yeah, totally. But again, what the hell would a completely insane Grey Knight be like?

[troll mode] Maybe he would think bathing in the blood of Sisters of Battle will protect him from evil nanobots, and that killing 1 billion Imperial Guard because one of them may have told people he saw a daemon is a good idea…[/troll mode]