What will happen when Donny Most rises up from the mist?

This only seems to happen once a century and I'd like to know why this is the case. Is there some kind of folklore behind this? What does he do once he rises from the mist? How long does he stay risen? When has he last risen so we'll know when he will rise again? I'd like to know because it may not be a good idea for me to miss Donny Most rising up from the mist. There are just so many questions that need answering here!