Hm, thats actually pretty good!
I think one of my favorites would be for me (or victim if I was another participant) to be bent over with my head on a chopping block or stump while being fucked from behind and jacking myself off. The third guy would have the axe standing over us. He could be jacking off as well. We would have practiced coordinating our orgasms so that me and the guy fucking would cum at the same time, just as we were shooting the headsman would bring the axe down cutting off my head cleanly and quickly. He could then do one of two things, either pick my head up and use it, either end, to finish himself off...or if he wasn't close enough or wanted more then the 'fucking' guy would be next for the block. Naturally he didn't expect that so he would have his hands tied and be forced down...suddenly being in the position of the guy he was just dominating makes him really hot so he comes a second time...just as he does *chop* off comes his head too. Finally thats enough for the headsman so he cums on both of our faces.
Hehe, sorry not a writer but fantasizing sure is fun!