User blog comment:DirgeOfCerberus111/Adeptus Custodes Vs. Grey Knights/@comment-2253300-20111028201241/@comment-3258762-20111028202446

However, while GK's fight with these powers and are loyal to the emperor, the Custodes will never break AT ALL. The GKs could do that, but while GKs do not feel fear, the Custodes feel nothing of their own lives. In other words, the Custodes (who with good battlegear equal the GKs) are the Imperiums equivalent to Necrons.