You need the key from a monster that you slay, I forgot the name but it's a hard one to kill and it has a chance to drop the "corridor key", it's name is written in purple text and let's you continue with the game.
Like i said in another reply, be kind to all characters then head to the dungeon in the basement and free the crying orphan using the key you get from doing the kings quests and then beat the game from there.
