Originally posted by AllOutWar76:

I'm guessing beating the second boss triggers it. Now that I think about it, I first encountered the black forest mobs after beating the first boss even though I hadn't been there yet either. So maybe it's the games way of giving clues on where to go next.

Yeah, it's the bosses that trigger certain attack types. You'll get entirely new attacks every time you kill a new boss, I've noticed they're generally from the biome you need to work on next.