Embers of His Past is the last quest in a sequence of Fari-Wari quests that allow you to participate in the "Mercenary Camp" BC which previously was only available to those who went to the Fan-Fest events. In the BC you get to fight the 5 Serpent Generals.
If Flameserpent Galadar is imprisoned you cannot activate this quest.
To complete Embers of His Past you have to go to Mount Zhayolm at L-7 at night (this is near the Halvung entrance) and click on the Sprightly Footsteps, and then the Withered Petals on the nearby cliff. After the cutscene you'll be warped to whitegate. Now you have to get a Hydrangea; you can either buy this at the auction house (it's like 1k or so) or if you're feeling bloodthirsty you can go kill Hilltroll Warriors and Red Mages at Mount Zhayolm for it. Trade the Hydrangea to the Withered Petals in Zhayolm, then go back to Fari Wari to complete the quest.
After JP midnight go to Fari Wari again to get the Mercenary Camp Entry slip. This object works only one time and the quest is not repeatable so make your fight count!
You can bring 5 guests with you into the fight. The BC is at Stellar Fulcrum in Delkfutt's tower.