Curtain of the Hlutr
In order to contain the evil of Mertorthar, and to shield the developing races of the Scattered Worlds, the Scattered Worlds Hlutr created the Curtain of the Hlutr, a screen of thought and mental song which surrounds the Galactic Core to prevent accidentally entering the Gathered Worlds, as well as to prevent traffic out of the Core. This Curtain effectively isolated the Core until Mertorthar's war against the Empire of the Iaranor about 600 million BCE. By that time, the Gergathan had learned how to penetrate the Curtain.
Following the defeat of Iaranor, the Curtain remained as a one-way membrane, telepathically warning Scattered Worlds vessels away. The Curtain was lowered and raised again several times, most notably in the Core interregnums.