I believe this was an episode from a early nineties nickelodeon show, or at least that's where I first remember seeing it. This kid was talking to a spirit within a lake. The spirits face almost looked like swamp thing. When the kid accidentally blew a leaf onto the lake the spirit got really pissed off. When the castle the kid came from is attacked by invaders the kid begs for the spirits help and from the lake comes an army of mounted knights.