The Duke of Soleanna is a character mentioned in the Sonic the Hedgehog comic series and its spin-offs published by Archie Comics. He was the former ruler of Soleanna, a city-state in alliance with the United Federation, and the father of Princess Elise.
Ten years ago, the duke and his researcher were experimenting on the white flame that was Solaris, their patron deity, in their research labs in an attempt to harness its time-warping powers. However, they ended up splitting the creature in half, forming two malevolent spirits; Mephiles who was Solaris' power and intelligence and Iblis who was Solaris' fiery power and destructive nature. Without any other option, the duke used the power of a Chaos Emerald to seal Iblis within Elise, dying in the process, while Shadow sealed Mephiles.
With Elise and Sonic having gone back in time to the days before Solaris was experimented on and blown out its flame, thus ensuring the disastrous split never occurred, the duke's current status remains unknown.