From The Mana World
The Void is the inter-dimension space between The Mana World.
A few beings are known to have crossed the Void. Most notable examples are Xakelbael the Dark (also known as Zax De'Kagen) - to conduct experiments - and Golbenez.
At this point in time (November 2020), players have only been able to cross the Void by death with a Black Rose with then.