In Astria Ascending, speak with Egyl in Harmony Orchard after making Kaydin a Hunter to obtain the Birth of a Legend side quest.
Find the Noise in the Fire Country
Fast travel to the Path to the Sagittarius Temple in the Blazing Field then jump down to the east. Interact with the glowing red orb to trigger a fight with the Night Dancer. Defeat it!
Go back to talk to Egyl in the Harmony Orchard
Return to Egyl in Harmony Orchard and speak with him.
Find the Noise in the Earth Country
Fast travel to the Broken Bridge in the Grand Canyon and head west until you spot the platforms. Jump up them then head east at the top to find a glowing orb next to the broken bridge.
Interact with it to trigger a fight with Inspired Death then defeat it.
Go back to talk to Egyl in the Harmony Orchard
Return to Egyl in Harmony Orchard and speak with him.
Find the Noises in Air Country
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Rewards: Mebsuta x1, Valiant Cap x1, Gold Stone x1 and 5,000 Lums