The opening
The characters attend a sunrise program at Cahokia when a ranger called Morning Star notices an extra post standing in the reconstructed wood circle. Its shadow points between the marked seasons. By noon, a neighborhood appears beyond the park boundary where an empty field stood that morning.
The residents have documents, family photographs, and memories of living there for decades. Morning Star asks the party to help before officials arrive. Birds circle him in the shape of a figure found on old engraved shells.
What the party is trying to do
Find out where the new neighborhood came from and stop the extra season from spreading across the floodplain.
A route through the campaign
- Measure the new post while crops around it race from seed to harvest and back again.
- Interview residents whose histories fit perfectly into gaps in local records.
- Enter a chamber inside the great mound that contains calendars for years that never happened.
- Discover that Morning Star placed the extra post before inviting the party to the site.
- Confront his winged ceremonial form as the thirteenth season begins replacing the present.
Possible endings
- Remove the extra post and erase the returned neighborhood again.
- Defeat Morning Star and merge the two seasons without his control.
- Make the thirteenth season permanent and help both communities survive the collision.
History and invention
Cahokia was the largest pre-Columbian settlement north of Mexico. It included many mounds, planned activity areas, a palisade, and a circular timber monument often called Woodhenge. The calendar that selects seasons is a fictional change made for the adventure.
Content notes: flooding, food insecurity, displaced communities.
I ran something adjacent to The Thirteenth Season, but the party trusted the wrong faction almost immediately. I would keep that possibility open here too.
I would open The Thirteenth Season right in Cahokia on the Mississippi floodplain and let the party find the first clue before anyone explains it.
I can already hear my group arguing over whether to remove the extra post and erase the returned neighborhood again.
The Thirteenth Season feels playable rather than merely clever. I know what the party is doing, what can go wrong, and where I can improvise.
I would move the reveal in The Thirteenth Season one session earlier and let the players spend longer deciding what to do with it.
The choice to defeat morning star and merge the two seasons without his control would absolutely split my table. Perfect.
The part where the party must measure the new post while crops around it race from seed to harvest and back again gave me the whole first session.
I would give the first clue in The Thirteenth Season to the least scholarly character. It keeps the mystery from becoming one person reading while everyone waits.
My players would turn the first choice in The Thirteenth Season into a three hour argument. I mean that as praise.