This first thing we get to when we're reading this Slayers expansion is some really exciting new lore that helps flesh out the existence of the districts in the core book. The ideas here are really good and they don't break the implied setting or make it incompatible. Rather, the Void enhances the setting; and offering some permanence to the core districts allows you to easily adapt these new chunks to the setting.
We get an entirely new district, The Citadel, and the supplement puts this at the heart of the City. It's a great district and fits in well with the other districts of Slayers. The details provided for this new place to explore are evocative and full of adventure seeds—always nice to see!
Next we have the set-up for the campaign itself, and this is detailed well and easy to grasp. The series of missions can be connected in whatever ways the GM sees fit, with events being triggered as time passes to string everything together.
Now we get to the actual "session" break-downs, which are solid frameworks that you can apply like a template to various hunts. For the most part you need to provide the hunts themselves, but these are pretty easy to come up with for Slayers. In addition, there's some advice about using the hunts in the core book, which is a great start. These frameworks are an excellent way to help weave an underlying story through the "monster-of-the-week" style hunts that works well with Slayers. If your campaign is a tv series, the framework in Clockwork Void is the overarching plot that connects everything. (With an awesome season finale provided!)
Lastly, we get some quick rules to help expand the city and track it, and also a way for the void to consume the city.
Overall, this is a great expansion to Slayers that can help you create a larger plot between your session hunts. The way it is presented is easily digestible. The only critique I would give is that there are some reoccurring things that are designed to show up between various frameworks and I think if the supplement provided a little cheat sheet or scorecard you could fill out as you play, that would save me the 20 seconds of doing it myself. So not a major thing at all, mostly just an "I'm lazy" thing.
If you like Slayers but think that the "random" hunts are just a bit too random, you should really grab The Clockwork Void. If you want to create your own overarching plotline, you could use this supplement as a template for your own ideas. There's even ways you can *reuse* the framework over and over again.
I really hope we see more from Rhombencephalon because this is really good stuff!