i know the generally accepted opinion is that it would never happen, but is it possible to fall to chaos? after all, Grey knights are technically psykers (which i guess should open them up to the taint of chaos), and Hammer of Daemons does a whole spiel on a grey knight getting possessed by a daemon.

I think a Fallen Grey Knight Sorcerer would be kinda cool.

Officially,no. According to the current background material, they are ultra-superpure & Wardtastic in extremis. Personally, I have never liked the idea of something, especially an entire group, being uncorruptable. I think it does a disservice to the setting & the horrific nature of Chaos that is written into the background of 40K. In my games, nothing is immune to Chaos. Some are better able to resist/handle it, but never immune. Just my opinion, take it for what it's worth.

 i would say Nurgle all the way…

 All according to plan…

Santiago said:

Why not?  They already worship a half dead rotting corpse anyway.

I seem to recall a quote somewhere in one of the books that "no Grey Knight had ever followed the path of glory" or something along those lines.  So the implication would be it might not have happened… yet. 

Of course, I would imagine that if such a Grey Knight did fall from grace in such a spectacular manner, he would bring such shame on the Chapter that they would take very extreme measures to purge that shame. 

I could guess that all the fallen grey knights have been killed, either by daemons they are facing or their comrades, thus leaving no record of it. I am not too familiar with grey knights, except their 666 method mindrape training…

If they were truly incorruptible, they would not have needed to bathe in sororitas blood to protect themselves.

They're also really big on leaving no witnesses.

To me, that says that incorruptible is an OFFICIAL stance on the matter; its what their papers state when they're doing budget applications.

In reality, I'd be willing to wager the whole secrecy bit has a lot more to do with making sure word never gets out of the one marine every other mission that explodes into a chaos-spawn from just too many gifts.

It could just be a play on words.

Any Grey Knight who does fall ceases to be a Grey Knight (in past as well as in the present). That is to say, any Grey Knight who could fall was never really a Grey Knight to begin with, no matter what their armour, weapons, and training looked like.

or one as decided to fight fire by fire and uses the weapon of chaos against chaos… rejected from the order, hunted, etc, he still is blazing a trail of destruction within the Eye… never understanding how much the gods enjoy it…

Alpha Chaos 13 said:

I'd say this is quite clearly untrue. Considering the Grey Knights have been known to bathe in the blood of the innocent, its pretty clear they are already corrupted, either by Chaos or by Matt Ward's writing.

They may have all fallen already, and its just being covered up.

Seeten said:

"ultra-superpure & Wardtastic" refers to the stuff he wrote about them being able to wield demon weapons without risk of corruption & generating psychic holy flame of purity. That crap. I'd say corrupted by Matt Ward's writing.

 Current fluff suggests no, but as others have said that's somewhat debatable as fluff is ALWAYS written as seen from inside the universe and thus might be false. I agree that it's stupid to have a large group of humans are incorruptible. Individuals? Yes, possibly. Xeno's? Certainly, by their alien nature being immune.

It's a Matt Ward'ish fluff, I'd honestly say f*** it and do what I felt would make a great story.

In my version of the universe, it doesn't happen. Though, I also ignore all most of the more current tabletop fluff, so take that as you will. I'd rule the same way for Exorcists due to how their psycho-indoctrination works.

The official stance, both in-universe and without, is that no Grey Knight has ever fallen to chaos, and it's not a clever use of wording or some other loophole. Read up on what the Grey Knights go through: besides mindscrubbing, they get hexa/pentagrammic tattoos to ward off Chaos, live lives of ridiculously monastic purity focusing entirely on destroying Chaos, etc. 

Yes, the Matt Ward bathing in blood thing is stupid, but even then, that's technically not being corrupted, it's using the sacrifice of the Sororitas to achieve the Emperor's will (YES IT'S STUPID, LET'S PRETEND IT DIDN'T HAPPEN). 

Grey Knights are canonically the only thing that can't (or at least haven't) fallen to Chaos, so if you did make one, realize how much of a GIGANTIC deal this would be, probably to the point that the entire Chapter would prioritize hunting this heretic down. 

HTMC said:

Don't Psychic Blanks and Tau can't/don't/haven't ever fallen to Chaos?

Star Child either are immune to chaos.