In the chapter 356 "Tartarus Prolouge" Kyouka noticed that Silver is in his "human" form. That means Kyouka has seen another form of Silver before. I suppose this is confirmation that devil slayers can transform into the devils and Silver did it in the past a lot of times. Yet "black marks" of what Grey showed didn't changed his "form" at all, so I suppose that it will lead to fully demon transformation like Etherious Form with some inhuman features like horns and claws. It's a pity we didn't see Silver transformation. What do you think about it?