Woah. i was in the tree of life and i saw a building so i went in and there was i rank 10 boss. 8,000 health! was I not supposed to see this? anyway I got a badge called Sword Saint.
You stumbled across one of the 'secret' bosses. It's not that you weren't supposed to see it at all, it's just that unless you look for it, or explore, you miss bosses just like this. Congrats on your badge!