When the Dragon Empress victoriously concluded the Gods War, one of her first tasks was to seal the Sea of Stars away from dangerous influences such as the distant heavens and the various hells. Now, such a seal could not be total but it could make travel between the realms, especially travel into the Sea of Stars, much more difficult. If people wanted to flee to the heaven or escape to the lower realms, the Empress did not care, it was things coming out of those realms to disrupt her rule that concerned her.
In addition to the wards that protected the realm as a whole, the Empress dispatched agents to locate such phenomenon as hellmouths, stairways to heaven, gates to hell and other such geographies that allowed otherworldly being to impinge on her world. Slowly, over the course of a century or two, all of the major portals were located and sealed or destroyed. A small number of minor portals, especially those that operated on a cyclic schedule, survived being either overlooked, lost or -in a very few cases- deliberately hidden. One of the cyclic gates to the underworld would turn out to have been hidden under the Summer Palace of the tyrant Ba’ai’i and it was sealed at the end of the Nightmare War that destroyed his Dark Star Dominion.
Sea of Stars RPG Design Journal 