Image: Sucker Punch Productions / PlayStationOfficial screenshot from Sucker Punch Productions / PlayStation.
Dirge of the Fallen Forge asks you to play a flute song at a grave. You need Lament of the Storm, unlocked with 5 singing crickets, and the grave opens after A Reckoning in Blood.
Collect 5 singing crickets to learn Lament of the Storm. Equip it under Gear, then Accessories. After A Reckoning in Blood, go to the grave on Yarikawa's east coast and play the song there.
Crickets sit in cages in cemeteries. Listen for chirping to find them. The trophy pops almost as soon as the song starts.
The grave belongs to Taka, Yuna's brother and the blacksmith you rescued in Act 1.