This quest is available upon your first visit to Balterossa. You also have to hire Caedmon and become more friendly by talking to him outside of the guild in Balterossa. As with any of the leaders with a parameter bonus, you need to exit to the world map and back again several times before his red speech bubble appears.
He appears next in Cafe Moondust and initiates the quest.
Spoiler warning: This section may contain information that could spoil the story line or displays in depth quest or fight details.
Pleasant Young Man: "Seems the Jhana have been popping up in the Great Sand Sea. You wouldn't think they'd have forgotten being blasted by the Gae Bolg already... ...That's the story, anyway."
Bartender: "Well... Once you quit trying, your life is over. How's it different for beastmen? If you've got a goal, keep trying until you reach it!"
Pleasant Young Man: "That's great, but not when the goal is destroying the city!"
Bartender's Daughter: "Seems the Jhana have been reappearing in the Great Sand Sea. You wouldn't think they'd have forgotten being blasted by the Gae Bolg already... That's the latest news, Daddy."
Bartender: "The Jhana have never done anything but harm people... Yes, the Great Sand Sea is too dangerous. I forbid you from going there, beloved daughter!"
Bartender's Daughter: "A-alright..."
Caedmon: "The Jhana have grown too large in number. We of the Silver Falcons will go to Blackdale, lure them into battle and crush them. Will you lend a hand?"
Rush: "Sounds dull."
Caedmon: "Of all beings that deserve mercy, the Jhana are not among their number."
Caedmon: "I would like to proceed to Blackdale as soon as possible. Are you prepared?"
Rush: "I need two secs..."
Caedmon: "If you have other pending matters, so be it."
Caedmon: "Glory to the Falcons!"
After we crush all the Jhana...
Caedmon: "Well done. They will behave themselves... for now. Fear is not an emotion they sustain for long periods of time. Here, take this. I may ask you for assistance again in the future. Until then."