Veil

The Veil ("Srud" in Tunge) is an aetherial barrier separating the various Anþerwelts or Mundi from each other. However, the Veil is not entirely impenetrable. With enough force, entities can pass through it. This allowed ancient human witches and wizards to summon spirits from Anþerwelts in order to work magic.

After Millenia, the Veil eventually became porous, allowing energies from the various worlds to permeate into others. No matter what one calls this energy; Ley, Mana, Qi, etc, some humans are able to use it to cast spells and other forms of magic.