This I something I specially made for Halloween. I recently learned what the blend tool was for a different project, and wondered if I could 'draw' with it. So I thought of some Halloween things that represent the day, and figured a jack-o-lantern, ghosts, a black cat, a zombie, and a bat in the full moon do the day some justice.
For making all of these, it was just looking at some reference photos to get some inspiration and then drawing two paths with the pen tool and using the blend tool to fill the gaps between them. Some of the paths are a combination of complex shapes and some use clipping masks to hide the blend tool at that point. Then it was just messing around with the stacking of layers and some colors to have the lines read well.