I currently on chapter two and did tbe first one to appear. But I couldn't find the upgrade option for the mission.
It is the judged people's one. That is exactly what I discovered. That is a significant amount of text.
You will find enemies marked with golden visual effects under their feet.
Kill them you will get progress to unlock/upgrade seasonal mission.
But if you can't kill them in time, the visual effects will turn to purple effects, then killing won't add progress (if I remember correctly)
Read the season 5 related parts in compendium for more details.