Board Thread:Warhammer 40k General Discussion/@comment-28383085-20170330013226/@comment-26420219-20170330192857

Well Number 1 would go against everything we know so far. I’ve reread the original novel-series on the Grey Knights (Grey Knights-Dark Adeptus-Hammers of Daemons) quite recently and it clearly states that  no Grey Knight has ever fallen to Chaos, even under the greatest pressure.

For instance, in Hammer of Daemons, a Justicar named Alaric is captured on a Daemon World and the sorcerers there do succeed in breaking his Aegis barrier by using a ritual and a chaotic artefact and go on to imprison a daemon within Alaric’s body to corrupt him. Therefore it could be theoretically possible for a Grey Knight to be corrupted by Chaos. However in the novel, Alaric let the daemon take control of his ody and becomes a gladiator-champion, while preserving his identity  through his psychoindoctrination and faith in the Emperor. Once he is strong enough, Alaric reads the daemon’s mind, discovers a way to free himself of the daemon’s taint, takes back control of his body and exorcises he daemon from his own flesh before escaping the Daemon World. But having harboured a daemon in his flesh for several months, his Grey Knights brothers refuse to take Alaric back, regarding him as tainted. It is then heavily hinted at that Alaric eventually becomes an Inquisitor of the Ordo Malleus

Likewise Draigo was lost in the Warp for millenia and still stayed true to his oaths to the Emperor, but 2 seems entirely possible. Killing a radical Inquisitor could also do the trick.