Yoth was a Class H planet located Oralian sector, once thought to be a mythical planet. (Star Trek: Pioneer)
In late 2383, the Romulan Warbird IRW Fierce Talon disappeared over the surface of the planet. The USS Pioneer and the USS Stefansson were sent to investigate. While their the crews discovered a race of small rodent-like people in a vast hibernation chamber. Somehow their presence activate the wake sequence, and soon Captain Benjamin Kelsoe was face to face with Magistrate Molelink his people once belonged to something called the Greater Ha'n Supremacy. Kelsoe asked the USS Atlantis to come and assist them. Along with Atlantis came Kelsoe's brother, Julian Brooke, and his colleague Dr. Lilith Adar. While examining the ruins of the Youth civilization, the archeologists and crew found the statue of a deity known as the Beast. Meanwhile, Kelsoe and his crew located the two surviving Romulans with the reluctant help of Magistrate Molelink. Next to the Romulans was an Oppressor Gateway, which Dr. Adar accidentally open. As a result, the Beast awoke from his slumber and left Yoth. ("The Forgotten Planet")
The Tal Shiar had been lead to believe by Dr. Felix Adar that Yoth was the Oppressors homeworld. ("The Forgotten Planet")