"Final hit of Basic Combo Attack deals 20% more damage."
Acquisition
The Crescendo Ring can be found in a Chest at Subjugation Stronghold. (Map Link)
From the Bonsai, head through the concrete halfpipe and then up the ramps of the stronghold. Once inside, head to the far end of the area and turn right to find the Chest.
Crescendo Ring is not directly dropped by Enemies.