This whole post is the result of a conversation on FB, in the group Educational DM. It's a pretty neat FB group. Make use of this post as you will.
Voidstone is a rare metal found only in the depths of the earth. Voidstone - sometimes referred to as Nullstone - is notable for its strong anti-magic traits and its density. Anti-magic as in magic ceases to function entirely when near Voidstone.
It has been suggested that a large enough concentration of Voidstone could render a deity powerless.
A few clans of deep dwarves, both Derro and Duergar, know how to work Voidstone ore and alloy it with mithril, adamantine, and cold iron to create Voidsteel. As magical fires, such as those provided by captive fire elementals or dragon's fire wink out of existence near the Voidstone, these smiths use magma channeled up from the planet's core to first smelt the ore, and then to work it.