Re-watched the LotR movies (they're how old now?!), felt like drawing one of my favorite bits that didn't appear in them: the Balrog's "thing of slime" form it takes after its fall off the bridge. A captivating idea, a creature of fire and shadow transforming into sludge when it hits water, instead of just being snuffed out.
And while I'm looking to the book for inspiration: wings made of shadow-stuff. The original novel is famously unclear just what's going on there....
