OpenWrt is an embedded Linux distribution optimized for small routers and access points with minimal amounts of storage to work with.
NixOS is a general-purpose Linux distribution built from the ground up with declarative configuration in mind, usually using a bunch of storage to do its thing.
dewclaw is what happens if you try to mush the two together even though you know very well that you shouldn't.
Originally created by @pennae, you can find the annocement thread here.
This is a fork based on the origial code.