The Ancient
This is not an adventure but if you are a DM like me¸ I tend to do multiple one and done adventures. This will help with transitions in between adventures. So you can use one-off adventures and have a campaign or an overarching goal for your players. The Ancient is a master of time and knowledge¸ can take the form of many shapes and sizes and lives in a massive library. Oh¸ I forgot to say the said library is floating about his island that is in his own plane. The DM can really play with this character and help transition through adventures while having a lot of fun while doing it!