Sound-Keeping Tacet Core Development Materials Lampylumen Myriad's drop, used for Resonator Ascension.

The Tacet Core of Lampylumen Myriad, freezing the humming whispers. The surrounding sounds will be captured by the Tacet Core and turned into a cold absolute echo.
Sources
Drop: Lampylumen Myriad
Lampylumen Myriad - Early Access