The Re-Gou Ruby was a gem that was said to have great magical powers. It was protected by the dead Pharaoh Hotep in the Egypt subtheme of Adventurers. The Adventurers team and the villian Baron Von Barron are both trying to retrieve it while Hotep is trying to keep it for himself.
The ruby re-appears in the 2020 Creator 18+ set 10273 Haunted House. The backstory for this set apparently confirms that von Barron ended up with the ruby, but upon completion of a portrait depicting him with his prize, he wound cursed by Pharaoh Hotep and driven mad. Von Barron also attempted to extract the power of the ruby using a resonator, but only ended up summoning two friendly spirits that haunt his manor.