Xagyg Dust Nebula is a long, massive nebula that divides The Frontier from the Rim. Navigation is possible only through the Capellan Pass on one side of the Frontier, and the uncharted Solar Major route on the other side of the map - such routes were used by the Sathar during their invasions of the Frontier.
The name is an anagram for Gygax: the co-creator of Dungeons & Dragons and co-founder of TSR, Inc.