I've searched thoroughly in the wiki but I haven't found this set piece listed in the set pieces page or anywhere else. I apologize, I don't have a screenshot of it, as I searched for it after I had died and reset the world.
In the swamp biome I found a skeleton that had 1 piece of rot next to him and a chest. I opened the chest and inside I found 5 Ice Staff Blueprints. The chest puffed some smoke and my character said "This didn't work as intended" or something along those lines (I had the Viking girl). The trap didn't trigger anything obvious. Nothing attacked me and nothing seemed to have spawned. The game remained on Autumn (it was sometime in Autumn's first week) and changed seasons normally. No other weather effects. Nothing happened to my items, my sanity/hunger/health or my camp. I didn't notice any differences in the immediate vicinity, although I hadn't really explored it thoroughly earlier. I died in winter during an excursion to a McTusk hut and reset the world (so unrelated death to the chest).
I think this is a "rot trap", and pretty sure it is present at the Set Piece article. The effect is that items are instantly advanced to the next spoiling stage (Fresh to Stale, Stale to spoiled, spoiled to rot). Perhaps you didn't notice that, it's not such a huge effect.