Spellvoid is a Rank A Sword forged from Mithril.
|Fused from 3||Seal of Magic|
|Attack Points (AP)||14|
|Defense Points (DP)||16|
|Sell Price (Sword Coins)||20,000|
|Sword Contest Points||210|
As you can see in the manga, a spellcaster calls forth a fire spell. When it is unleased onto the other fighter though, rather than being harmed by it, his sword voids out the spell and renders it useless.
The wielder of Spellvoid in the manga is Ilec, the legendary swordsman. So it would make sense that he would use such a strong sword.