Originally posted by Zep Tepi:

One of the runestones in game claims there are seven bosses. We're up to six now, but still have two biomes left. So either the count was wrong, or the far north boss is not counted among the other seven, which means it could have a special means of unlock?

That is an excellent point and I did not remember that until going to the wiki. However, in the wiki there is a rune in the Mistlands that says:

"Trust nothing in the mist. I passed through here in a group of seven people but every time we stopped to count, we counted eight. We do not know who the eighth was, but by morning they were gone."

Now this is most likely a reference to Odin as he likes to do his disappearing/reappearing thing but perhaps it's in reference to a secret boss? You're right. The number seven does come up a lot in the runes and I had not taken that into consideration