On this page of our trophy guide to Ghost of Tsushima you can find a detailed description of Dirge of the Fallen Forge trophy. You can get this trophy for playing a Lament of the Storm melody on a flute during the funeral of your friend.
The following description contains some major story spoilers - BEWARE!.
This trophy is associated with the mechanics which allows you to play different melodies on the flute and the whole thing was described in more detail on another page of our guide - Playing melodies on the flute - what is the use of this?.
As a reminder, by moving your finger to the left on the controller's touch panel, Jin will use the flute. Unfortunately, this trophy forces you to unlock the second melody available to play on the flute, which is unavailable at the beginning.
To unlock the Lament of the Storm melody, you need to find cages with crickets, which are one of the secrets / collectibles in the game. Look for them around the graveyards. Unfortunately, cemeteries are not marked by any question markers on the map. You have to collect 5 random cages of the 20 available in the game world.
Find the flute in the hero's inventory - check the Gear tab and set the Lament of the Storm melody as the active one.
To unlock this trophy you have to wait until A Reckoning in Blood main quest. In the final part of this task, Jin and Yuna will meet at Taki's grave. That's when you can play a tune and earn the trophy.
If you couldn't unlock the Lament of the Storm before completing A Reckoning in Blood main quest, then you can return to the Taki's grave later in the game. It will be marked on the map of Tsushima.