History doesn't repeat itself

Is your goal to face a legendary monster? Well, that is usually suicide – if you can find one. But if your guild would actually manage to kill one, it’s dead. Gone. Rare monsters and quests do not resurrect or come again, whether it’s an ancient beast, a crumbling temple, an island rising out of the sea or a great ritual that fails or succeeds.

You might get away with an item never seen before, or the unique scar that tells about the experience. There are not many of these opportunities but they are usually important enough to affect the whole world and its history. All you have to do is be at the right place at the right time.