After you are sufficiently geared, Warin asks you to exterminate some coblyns.
This quest is only available if you started the game in Ul'dah, you have completed the quest Prudence at This Junction, and have reached level 9.
Starting the Quest
Talk to Warin in Central Thanalan to start the quest.
Lure Out and Slay Coblyns
Warin will give you a Silver Ore Cluster. It will be in the Key Items section of your inventory. Follow the train tracks south to the next quest destination. In the tunnel, you will find a Narrow Fissure that is glowing purple. Interact with the Narrow Fissure and use the Silver Ore Cluster from your Key Items, or target the Narrow Fissure and click the Silver Ore Cluster in the list of quests on the side of the screen. A Silver Coblyn will attack. After you kill it, two more will attack, so slay them as well.
Report to Warin
Go back to Black Brush Station and talk to Warin, who has a green checkmark over his head, to complete the quest.
Next Main Scenario Quest
After completing the quest, talk to Zuzumeda to accept the next quest: Way Down in the Hole.