Scars of Faerûn
The Slaying Stone is an artifact of supposedly godly origin. Along with the Totem of Amarrindam there are two of the seven items needed to assemble a divine device, capable of simulating an effect akin to the Spellplague, though this has yet to be proven yet.
Some people go as far as to claim that such a device was used to temporally disrupt the Weave and allow the opportunity for Cyric and Shar to assassinate Mystra, which in effect kickstarted the collapse of the Weave and subsequently created the Spellplague.
It was secretly kept in hiding by the dragon Tyristis, out of spite towards her mistressTreona.
The Slaying Stone emanates an arcane energy that appears to have blue and purple hues when is near another item or place that is somehow connected with the creation of the divine device.
There is not a lot that someone can understand by the stone itself, as it seems it can only reveal signs of magic when it is somehow triggered by an event.