It appears to have been crystalized from a pool of dark water, which is what had originally been there. ( which does not rule out someone transforming that water into the mirror, but a mirror at least was not brought to it. whoever did this clearly had a great deal of power. ) Few could reach the area it is currently at.

As for the Beastmaster, I am not certain. If he is after the power behind it, I imagine it would be of interest to him, but the earthquakes appear to be cracking it, destroying it. Perhaps he realized, and he is here to collect when it's released.