Its all in the title. Who or what is the bane of your adventure. It could be anyone or anything. They could be something you HAVE to kill or someone you hate. They could of done something unforgiving, making you just want to destroy everything to do with them.
My sworn enemies are the pathetic goats from the land of Skyrim. Don't ask why... I just can't let a goat run past me without having and axe to the face. Though I'm definitely sure I have the record for most goats killed in a playthrough.
My Arch Enemy is that Frost Troll that is always on the path way to the GreyBeards. When I first played Skyrim and went up those steps I ran into it and kept getting killed. I eventually had to go to whiterun, get a follower, and give them a bow with a fire enchantment. To this day I always come prepared with weapons with fire enchantments or fire spells. That thing is a monster for people who are low leveled.