You must have completed Red Mage Artifact 1 for the Fencing Degen before you can get this quest.
This quest begins in Upper Jeuno. Visit Guslum in the armor shop. After you receive the quest from Guslam go to The Eldieme Necropolis and kill Tomb Warriors, Tomb Mages and others to get the Coffer key. After obtaining a key, search the zone for a coffer chest. Trade the coffer key to the coffer chest to receive some old gauntlets (key item).
After you received the old gauntlets, return to Guslam who will tell you that the gauntlets are work of Borghertz. But he cannot repair them.
You then have to go speak to Yin Pocanakhu in the Tenshodo headquarters in Neptune's Spire in Lower Jeuno. She will talk about a toolbox. You have to pay her 1000 gil to tell you where the toolbox is. After paying Yin Pocanakhu, she will tell you she left the toolbox on top of a crate outside the Duty-Free shop in Port Jeuno and that you can have it. The Duty-Free shop can be found near the Auction House in Port Jeuno.
Outside of the Duty-Free shop you will see a "???." Examine the ??? for a cut scene. Borghertz will appear and tell you to bring some shadow flames from Castle Zvahl Baileys and to leave it and the gauntlets inside of his toolbox.
Head to Castle Zvahl Baileys and examine the torch at F-8. A NM bomb by name of Dark Spark will appear. Kill it and examine the torch a second time to get the key item Shadow Flames.
After you received the flame return to Jeuno and re-examine the ??? outside of the Duty-Free to get your Warlock's Gloves.