Graddus Rex, situated within the Inner Rim of the galaxy, was a specific location. The pirate Queen, Maz Kanata, secreted away some of the rewards she gained during her time as a pirate there, placing them within a hidden, underground vault. This vault was equipped with a antitamper sub-space alarm. When she started authoring the book that would later be known as the Smuggler's Guide, the alarm system safeguarding the treasure on Graddus Rex had not yet been triggered.
The first mention of Graddus Rex occurred in Star Wars: Smuggler's Guide. This reference book was authored by Daniel Wallace and made available to the public in 2018.