I'm creating this topic to bring a secret boss to your attention, in case you guys missed it.
The boss event is called "Summon Stones", and he's in an area in Twigroot Forest accessed through Forgotten River. The reward was a book that granted +1 skill point.
Here's the boss fight video by me.
And how to find it, by another youtuber (TheDouken) who was kind enough to provide directions.
---
Feel free to share more secret bosses (if there are any). This was the only one I found so far, and I'm currently in Crystal Mines. I'm afraid I may have missed more. D: (Haven't been to the Cemetery nor Pyramid yet)