Void is essentially damage that comes from rewriting reality or manipulating the stuff of creation/the Void (reducing things to the primal Void and rebuilding them from that infinite sea of possibility into what you desire or just reducing things to that primal void and letting reality reassert itself (which will put things back together just in a certain amount of agony) or dropping raw Void energy onto your opponent and watching as reality melts. It's all elements and none at all, everything and nothing, the absence and potential of all things, the moments between time, etc., raw mana is of this element.
Combat-wise, void is not very useful in most situations as void resistance is usually at 100%, and a monster's other elemental modifiers are almost always higher than their void resistance, meaning they have over 100% resistance to an element. Void damage should not be used unless a player is missing some weapon elements or a monster has very low elemental modifiers such as the Zeel. There are few ways to deal non-elemental damage to a monster, and the ways to do this are only at mid to high levels (level 70-150).
All monsters have a void resistance of 100%, save for a few exceptions like Absolix and Carnax. There is no way to have constant void defense from monsters, but there are some ways to protect the player with items that lower the damage by a special like the Nemesis shield or the full set bonus of Horo-Show Void Vigilante
The Dragon Blade (acquired from ''.Quest for Dragon Blade'' does void damage to dragons at an elemental modifier of 200%. As of recent updates all monsters have a void elemental modifier of 200% save for a few such as The Galin, Nightbane, Abolix and Carnax.