User blog comment:DirgeOfCerberus111/Adeptus Custodes Vs. Grey Knights/@comment-64.187.66.176-20140214022155/@comment-79.193.1.205-20150326175715

I think, you have to be exeptional in any way, practicly a saint only to be considered to be recruited into the custodes. The monk orders, wo supply the legio with  recruit surely sent them on crusades, where they have to prove themselves and collect experience. Also a part of the Custodes, called the emperors companios are over 10000 years old and have fought wiin the unification wars, the great crusade and the horus herisy. One of these fellows can kill a squad of paladins with his little finger. once recruited, a custodes studies the countless arts of death, they are meant to eb bodyguards, but they are also assasins, hunters, master tactitians and much more. They spent their lifes with fighting each other, not for decades or centuries but milennia. A fight between such wariors is never a game, even though it is no fght between enemies. Geneticly a custidian is closer to a primarch, than to an astartes. The grey knights are focused on recruiting strong psykes and train them mainly to oppose the touch of chaos. Their trainig is far briefer and less selective than the ones of other chapters, they dont even have scouts. You can count on one thing: the training to become a custodian is the equal of 1000 wars. There are millions of possible cnadidats but only a few dozend free places every millenium.