Sine Nomine? More like Sine Qua Non, amirite? Kevin Crawford has been killing it with these OSR sandbox supplements, and Silent Legions is no different. Each SN setting tends to add a couple innovations over old-school mechanics, but nothing too radical. Silent Legions is no different, using the "slaughter die" to make combat a significantly more dangerous affair - at least for human beings (lots of monsters are immune to the slaughter die). Other than that, there are evocative setting details, and scads of inspirational tables. That's the Sine Nomine way.
It's not always easy to capture what makes these products so brilliant. I'd say it's all in the details. Crawford doesn't just churn these out, although it would be easy to do so with the Sine Nomine formula (maybe he has tables for making tables, somewhere). Instead, everything here is very quality and professional. There's just the right amount of detail. The prose gives you just enough detail to put the flavor of the setting in your head, without crowding out your own ideas. The tables themselves are uniformly excellent - one gets the impression that Kevin actually uses these. His kickstarters meet their goals and deadlines. The man is a machine.
Anyway, Silent Legions has all you want for your own Mythos-inspired campaign. The idea of having the GM come up with his or her own cosmology is actually a pretty radical idea, when you consider that Lovecraft has been appropriated not just into games like CoC but even lots of sword and sorcery settings. Who would have thought to ditch Cthulhu as a starting point for a Mythos setting?
There are plenty of tools in here that help not just to define your own personal Mythos, but also to build secretive cults, magical artifacts and plot hooks aplenty. I'm curious to know how a sandbox campaign would work in such a setting, but if that's not your speed, you can still get a lot of use out of this product whenever your own creativity runs a little dry. If nothing else, you can just read this stuff, and get inspired.
Suffice to say, I recommend Silent Legions without reservation.
[5 of 5 Stars!]