@oshizu, happy to help, and I do love cookies.
What's a dynasty challenge?
Every year, the Challenges Team creates a new multi-generational dynasty challenge that lasts until certain requirements are met.
To continue and ultimately complete the challenge, the founder(s) and their heirs must achieve specific goals.
So far, there's been an Immortal Dynasty Challenge (2014), Rival Dynasties Challenge (2015), 10-Gen Townies Challenge (2016), How Many Life States in Your World? Challenge (2017).
I'm sure a member of the Challenges Team can provide a more thrilling explanation, lol.
A new challenge is announced yearly in November. Here's a link to the rules for the challenges I've just listed.
http://www.carls-sims-4-guide.com/forum/index.php?board=113.0@sdhoey Help?