Sister Solaris gave you this Cimeroran Medallion as a physical reminder of your actions in Cimerora. Indeed, when you look at it, you remember the time you fought against the forces of Mot and the Talons of Vengeance in Cimerora in an experience called...
What Was Built Upon the Past
You travelled back to Cimerora and discovered the true identity of the letter writer: the fabled Dream Doctor. The two of you worked together to recover the idol that Mot was sealed in. Dream Doctor believed that if you were exposed to some of Mot now that you could return to the present and be immune to its power for a time. However, there was one another issue.
The idol needed to be brought to Astoria in the time of Cimerora. The Talons of Vengeance had appeared and were intent on destroying the civilization while recovering Mot for themselves. If this happened, then the fight in the future would be lost.
You recovered the idol and then fought against the remnants of Romulus Augustus and the Talons of Vengeance. In the end, you were succesful in repelling the threat and set the scene for Marcus Valerius, the guard captain in Cimerora, to set sail in a boat built by Daedalus and Marathon towards the Abyss.
Since Mot was in Dark Astoria, you can only assume that the journey of Marcus Valerius was succesful, at the cost of his life.