When I started out with this, it felt to me like an en dash or em dash would do the trick of properly conveying that the final section of the sentence is very important information, as that is to be interpreted as a grave threat. But I think I'm probably quite way off and that there's other more proper ways to convey that the final part of the sentence is crucial information. This is the sentence as it is, any suggestions? With swords interlocked, the brute dared to reach his flaky black hand for the pouch that was strapped to the flank—all while more stygian riders were gaining.
I'm with @ChickenFreak , why not make it two (or more - perhaps three short sentences would give better rhythm?) sentences?
I agree with the "two sentences" crowd. I'll take it even further. If you want the information to have real impact, give it a whole new paragraph. Bam!
It might be interesting to see some examples so that we can comment on them. I don't agree that long sentences should always be split up into shorter sentences. Sometimes you need a variety of sentence lengths to make prose read well.
If you're worried that impact is being lost with a long sentence, it's definitely wise to split it. But I'm with @BayView - if you really want it to hit home, put the important bit on a line of its own.
I'll also weigh in with the 'two sentence' crew. I don't think the sentence works particularly well at the moment as it's trying to convey two quite different peices of information (what's happening in the fight and the distance to the otherriders). I tend to try and stick to one packet of 'vital' information per sentence? But equally, I dont think you want a really short sentence to follow the first one (I tried at first and just couldn't get it to work how I liked, I just don't think the sentence regarding the distance to the riders has enough impact to be kept really short). I'd be tempted to word the sentence something like this (apolgies if I've reworded this more then you were looking for): "Swords interlocked, the brute dared reach his flaky black hand into the pouch strapped to his giant flank. The rest of the Stgian riders were still gaining ground rapidly, suddenly close enough he could count indivdual beads of sweat forming on their foreheads as they beat their mounts into a lather" It might have got a little over complicated (I tend to do that), but I'm trying to show how it can sound better when split out. Hope this helps!