What if a new mob was added, some kind of "nightmare", that attacks if you stay in a certain map too long? Or maybe it just wanders FT incessantly and looks for players to eat... Logging off/on should not restart the counter.. but this mob would perhaps be a little different.
You could not kill it in a normal fashion - weapons, spells, and pets will not affect it.
I can see a few possibilities:
- you cannot kill it at all, and have to run - this is a common theme in many video games
- you can only kill it with certain tactics - perhaps it's slow, or it'll pause next to you for a little while, and you have to hit a context button to scare it off somehow, i.e. shine a torch at it and it's scared away
if you fail, it kills you... and if it kills you often, well, that's a sign you're maybe not paying attention to the game *cough*autoclicker*cough*
this thing would eat autoclickers regularly, if designed properly

and it may add a little excitement to the game itself
as per my older idea, perhaps a small (random) reward could be given to those that defeat it