Disk Vol.21 "Armada Battle"
From Ephinea PSO Wiki
Disk Vol.21 "Armada Battle" is a playable music disk. It plays the song "Armada" from the game Skies of Arcadia, released for the Sega Dreamcast and Nintendo Gamecube (as Skies of Arcadia: Legends).
All music disks can be obtained by killing any enemy.