In astronomy, voids are the empty spaces between filaments, the largest-scale structures in the universe, even the continent of Tibia.
It can also be described as "A timeless, lifeless plane of nothing located on the edge of creation." There is a broken portal in the Necromant House that instead of going to the Pits of Inferno teleports you to this plane. You will exit the void through another portal that takes you near the SD quest north of Plains of Havoc, here.
Void can also be found in various other places in Tibia, like the Dreamer's Challenge Quest, in a mission of the The Scatterbrained Sorcerer Quest, the Soul Wells and the Shadow Nexus in the Demon Forge or around the lairs of Fahim the Wise and Merikh the Slaughterer.