The Flawed Stone
Available at the start of the Quest: The Siege of Bastion's Keep (Act III)
Unlocked after Kulle's Black Soulstone
Tyrael: The Black Soulstone was created to be an improvement over the original three. They were crafted to contain single demons, but Kulle intended his to hold many.
But the Black Soulstone is flawed and cannot contain that many demons for long. We must capture Azmodan within it and shatter it... lest all these Evils be reborn.
Don't know if it was brought up before, but how did Tyrael go from 'we must shatter it' to 'we must lock it away in a vault'?
Good question. It seems like an oversight on the part of the writers.
Adria claimed that she knew how to shatter in in such a way as to destroy (or banish, or whatever) the evils contained within. Tyrael had no reason to doubt her up to that point.