This is a Void Crystal used only by the orcs to communicate with their dead. This magical crystal is very rare and sacred amongst the Orcish Hordes.
Void Crystals are used to open the path inside the Void and reach Primus Magus.
How to Obtain
Drops from many bosses, including a high probability from General Azgalor
Wizards in the Valley of Sernos have a decent chance of dropping
Witches in the arcane mountains have a high probability of dropping
Can drop from creatures in Hell
Can be found through lockpicking Grand Chests, or even Lockboxes if Thievery is high enough.
And of course they drop from creatures in the void itself.