Veil

"This is it. Veil. This is where our world, and the Red Star Dimension, is being connected together. The Nazi Union helped many other Coalition of the Red Star member states to connect it with the Veil to ensure their will to become more stronger than ever before."

--Su Ji-Hoon, Into the Veil

The Veil is a barrier between the Red Star Dimension (instead of the Black Sun Dimension due to offensive content) and the "normal" dimension inhabited by humans, seen in Wolfenstein.