Libris Mortis. Which, now that I think of it, was the last time I had the opportunity to create an entire new set of monsters without any real restrictions other than theme.
(Numenera? If you don't know, go read this, then come back. I'll wait.)
So, yeah, the Numenera Bestiary. It turns out I get to write about half the creatures that'll be appearing in the product. The very first creature I created has the working name "spurn." (The spurn are, so far, the only monster in my concept collection that have an irregular plural form: 1 spurn and 20 spurn are both correct usage. Later editorial oversight could nix that.)
From afar, a gang of spurn seem nothing more than humanoids in oily rags. But up close, they're revealed as creatures formed of smashed machines, shattered synth, matted hair, drippy sludge, and garbage. Spurn are self-organizing collections of refuse that evolved among the landfills and dumps of previous civilizations over deep time.
I'm biased, but I love the concept. In the Ninth World, even "simple" dirt isn't actually simple. In fact, it's not even called dirt; it's drit, a fine, artificial soil made from microscopic pieces of ancient, unknown technologies and structures. Which makes drit fertile soil for creatures like spurn to spring up, so to speak.
I'm hoping to write about what I'm working on in the Numenera Bestiary in future posts, and give you an insight into some of the ways I'm approaching design. Until then, iadece!