Quest is "Ifrit Bleeds, We Can Kill It" which should be available from Thancred in Waking Sands at x:6.2, y:5.1 providing that you have completed "A Recurring Problem" quest and are a level 50 disciple of War/Magic. Providing you have matched the requirements and if Thancred is not giving you any quest then I would start by checking with the NPC "Swift" at the Hall of Flames in Uldah to see if they have the next part of the quest for you (may have the quest unlocked already?). It's also worth checking your quest log to see which quests are in progress - may need to abandon some of them to give it a "reset" for the quest to appear.