Skaia was a gem said to have several magical properties, with one being that it glowed in the hands of the Empress' bloodline.
History
Meeting Phantom
Centuries ago, Empress Aria began hearing stories about the legendary thief Phantom, who stole from the corrupt nobles of Maple World. Aria grew intrigued and wished to meet Phantom, but quickly realized that Ereve had no treasure worth stealing. In order to lure Phantom to Ereve, Aria began spreading rumors of a magical gem called Skaia. Sure enough, Aria received a letter from Phantom, who warned that he was coming to steal the gem.
When Phantom came to meet Aria, he instead decided to convince her to hand over Skaia, rather than steal it directly. Though Aria refused to give it up, Phantom persistently returned each night to ask for it. This quickly became a game for them, and over time, the two gradually fell in love.
Aria's Death
One night, Aria told Phantom that she had invited an envoy from the Black Mage to Ereve in the hope of negotiating a peace treaty. However, the conference was a trap, during which Lotus and Orchid killed Aria and ransacked Ereve. Phantom immediately rushed to the scene, where he found Aria lying dead beside a wounded Shinsoo. Shinsoo then gave Skaia to Phantom, asking him to protect it. Unbeknownst to them both, however, was that Skaia was merely a common blue gem that Aria had favored.
Stopping Hilla
Centuries later, Hilla attempted to overthrow Cygnus by posing as a noblewoman and claiming that she was in possession of Skaia, and that among its properties, it was said to glow in the hands of someone of the Empress' bloodline. Taking advantage of the fact that there were little to no historical records on Skaia's properties, Hilla enchanted a common red jewel to glow in her hands in order to substantiate her claim.
However, Hilla's plot was foiled by Phantom, who chased Hilla off by claiming that the real Skaia was in his possession. Phantom then slipped Skaia back into Cygnus' hands before disappearing, thus returning Skaia to its rightful owner. Eventually, Cygnus discovered Aria's diary among her keepsakes and passed it on to Phantom, who then learned the truth that how Aria had invented Skaia in order to meet him.