I was looking for help on this, and caught this post. I went back to the cave, and fought some green spider (lvl 13 rare enemy), but chaos ruler still doesn't show up on the craft list. Did I get the wrong spider? It was in Grimina cave, right? And yes, I already made chaos emperor.
Thanks, I figured it out. It was in a cave near the end of the plains that I'd long forgotten about. I was gonna come back to delete my post, but since you responded, that would be inappropriate. I'll just leave this up, in case anyone else gets confused about it.