The =I= symbol? Just try putting some greenstuff over the shoulder pad, and then cut out the shape with a hobby knife. Just try sculpting an "I" shape with two small lines coming off of it. If you want the skull, you could also probably take a skull bit, cut off the back half, and put it in the green stuff in the middle of the "I"
I'm guessing this is what you want-
http://images.wikia.com/warhammer40k/images/archive/a/a4/20120903081127!Inquisiton.png