Magister Alminas:
After I obtained the Resonance Amulet, I examined it in some detail.
While I do not know its exact composition, I do know that it was made from some material that Gran Kain cherished dearly.
Halisha, one of the rulers of Hell, is said to use this very same material. Perhaps you could get it by defeating his Minions or Archon?
Luckily, I have heard that Halisha is now sending many of his minions to the Four Sepulchers. If I were you, I would hurry there.
Hmm... the evil creatures in the Shrine of Loyalty are also Minions of Halisha. Perhaps they also have the material you seek.
Once you've obtained it, go to the Tablet in the southwest portion of the Wall of Argos.