Atroth is an old and forgotten elven castle. The elves had to forfeit the castle as another human kingdom attacked them. Years passed and the humans populated the castle Atroth until a calamity occured that devastated the surrounding land. With the land left barren the humans abandoned the castle leaving it to decay.

Now a Mage cursed by the demon that caused the calamity centuries ago tries to lift his curse and uses the castle as hideout, summoning the demon to undo his deeds and endangering the land anew.

...

I recommend using "The collected Monster of Sin" book for monster ideas.
It should be accessible for free somewhere, I think (or just be a pirate I guess *yarr*).