Magister Alminas:
I researched the Resonance Amulet on my own after I obtained it.
Its true character requires further study, but its physical structure is composed of one material favored by Gran Kain.
One of the Rulers of Hell, Halisha, is said to use this very same material. Were you to elminate a sufficient number of his minions, I imagine you could gain it for yourself.
Apparently, Halisha has sent many of his minions to the Four Sepulchers, so that is a good place for you to start.
Oh, the evil creatures at the Shrine of Loyalty are also Halisha's minions, if you would rather try your luck there. Wherever you go, if you eliminate enough of Halisha's minions, some who are more highly ranked will eventually appear, and one of those will likely have a Resonance Amulet.
Once you have it, go to the Tablet of Vision near the northwestern sector of the Wall of Argos.