Darksteel becomes protective when faced with failing, raged when faced with stupidity, ignorance, and a better competition, fearful when faced with nothing to do, and sadistic when overjoyed. So to make Darksteel afraid, just put him in a dangerous situation where he has no choice but to curl up WITHOUT introducing competition, danger to somepony else, or making him cocky. Silk is afraid of sleeping. [Note she makes an anti-sleep remedy for herself.] And nightmares.