Planet Earth exists as two sides of a coin, with one side containing humans and the other containing demons. These two realities are separated by a barrier that keeps both races from interacting with each other. Every 1000 years, a planetary solar eclipse occurs, which breaks down the barrier separating both realities. This allows demons and humans to interact with each other, leading to war and genocide as humans are slaughtered by their more powerful neighbors. This break in the barrier lasts for a number of hours, coinciding with the length of the eclipse. After the event ends, the barrier again raises and comes back into effect. Both races are forced back into their side of the coin as the two realities reassert themselves, separating humans and demons once again. Human civilization is then forced to rebuild its societies from scratch to repair from the devastation. However, the world remains forever changed after each eclipse, forcing humanity to adapt to its new environment.
Areas of the demon world continue to exist in the human side of the coin. These areas are separate pockets of reality that maintain a connection to the demon side and they exist in isolated areas (badlands, evil woods, etc), similar to the no-go zones in fairy tales with evil witches, mysterious dungeons, and corrupted magic. These contain the monsters that exist within the world of demons, as they are an extension of that realm. These beings cannot leave that pocket of reality and enter into the human domain. However, human beings are not bound by that restriction, and can seemingly enter these realms whenever they desire. This serves as the basis for quests in which adventurous and greedy explorers known as dungeon masters can face the dangers in these areas to collect ancient treasures. Most die horrible deaths or are never heard from again, but the rare few come back extraordinary wealthy or more powerful.
As it stands, humans can enter these pocket demonic worlds but demons are unable to do the same with human domains. How can it be the case?