The Voidhammer first appears in Siege of Dragonspear. After you collect the Voidstone Shards, Jegg in Boareskyr Bridge can forge this weapon or the Voidsword. This item can be imported into BG2EE, it's later found at Bodhi's Lair.
|“|| Voidhammer +3|
To all appearances a standard war hammer, one lingering long in its presence will soon discover it is far more than that. Specially made by Jegg Hillcarver of Bridgefort, this hammer has several voidstone shards worked into its metal head. A powerful weapon when used against most targets, its one great deficiency is in its utility against the undead. Objects of the Negative Material Plane have no effect on the unliving; neither then does this normally devastating weapon.