Secrets in the Swamp

From the introduction: This oneshot is meant to be used with 5e and is designed for a party of 5 of level 7. This oneshot can actually be added into any campaign as long as the group is travelling through a swamp. However, if it is run as a oneshot the group is sent to the swamp to investigate the recent disappearances of many travelling merchants. Once they arrive at the swamp they will find an abandoned caravan with signs of violence nearby. Eventually this will lead them deeper into the swamps where they will find a strange and ancient ruins. What secrets await them there?