I do love this topic, but I've been with my husband for too long to scare him the way some of the things we do ought to. He trusts me as much as he trusts himself, so I don't have any stories in which I managed to terrorize him, lol.
Once, he used a very cold blade on my neck and sliced a little too quickly. It felt just as though he had made a very deep cut. I actually stopped him (which I rarely, if ever, do) and put my hand to my throat to ensure that I was not bleeding out. I even got up and went to check in the mirror. That freaked me out a bit- but it really turned me on afterwards. :P
The last time I dominated him was when we were on a weekend getaway, alone. I lost control a bit. I whipped him fiercely, and he was more compliant than usual, which really turned me on. His preference is usually to play the dominant role. He lets me have my way with him, but it is more for my benefit than his. I do top from the bottom (whatever, I'm not submissive, sue me. :P) but this time he was totally passive, and obviously very aroused, which really excited me.
I had discovered a blade shaped like a scimitar in our hotel room, which was in itself somewhat novel. ( I am not a collector like some of the posters here.) I shoved him on the bed, tied his wrists to the bedposts, whipped him senseless with his own belt, and started slicing the blade all over his chest while I rode him. I broke his skin and didn't even realize it. I took the sharp tip of the blade, pushed it into his throat, and sliced it down to his navel, leaving a long, beautiful, angry red trail on his chest. I slapped him across the face multiple times with the sharp end of the blade pressed against his throat. That made me come so hard that I almost passed out.
In retrospect, he probably should have been scared, because I was not fully in control- but of course he wasn't. :P