Originally Posted by HalfTangible
Greater Daemons, Demon Princes, Primarchs, and fellow Adeptus Custodes. (All of the third that may end up fighting a custodes are either dead, incapacitated practically beyond recovery, a demon prince or missing at the moment)
I'm still wondering who among the Custodes has ever fallen to chaos. GK are the only imperial organization that's NEVER had a fall, so it might be interesting to see a custodes do so.
Bad, obviously. But interesting.
Also Kelbor-Hal, Arik Taranis, Eldrad, Malagor the Sigilight, Drago (by all rights this guy should be dead), Avatars of the Eldar Gods (Though those things are basically greater deamons anyway) and Vect.