In 19 BBY, Jedi Master Quinlan Vos was held at the prison where he was tortured by Count Dooku and various droids. Former Sith assassin Asajj Ventress and Boba Fett's syndicate later attempted to rescue Vos however Vos by then fully embraced the Dark side of the Force and spurned their rescue.
During the Clone Wars, Sith assassin Asajj Ventress participated in interrogations of various prisoners in the prison below Castle Serenno. Ventress later noted that no prisoners were released from the prison. Towards the end of the Clone Wars, Jedi Master Quinlan Vos was captured and detained by Count Dooku in one of the many cells located below the palace after he and former Sith assassin Ventress attempted to assassinate Dooku on the Outer Rim planet Raxus Secundus. While he was detained, Vos was continually tortured by droids who also monitored his sleep patterns and gave meals at irregular intervals. Dooku sometimes came to the cell where Vos was held and observed Vos being tortured or blast Vos with force lightning to hear his screams. Vos continually tried to press Dooku to talk in the hope that Dooku let something slip however this did not work according to Vos. At one point, Dooku interrogated Vos and sarcastically asked him about his mission. Although Vos answered truthfully, Dooku still ordered the torture droid to continue to torture Vos.
Ventress and Boba Fett's syndicate later attempted to rescue Vos. The bounty hunters distracted the various battle droids guarding Castle Serenno while Ventress sneaked into the palace. While sneaking into the palace, Ventress fought and destroyed several droids patrolling the inside of the palace. She later managed to enter into the prison where she met Vos. Vos however force- choked Ventress, enraged that Ventress was the one who lied to Vos about who killed Jedi Master Tholme after he used his psychometric powers when holding Tholme's lightsaber. Vos later fought against Ventress despite numerous pleas to not fully embrace the Dark side of the Force. This forced Ventress and the bounty hunters to flee the prison and the palace.
The prison first appeared in Christie Golden's 2015 canon novel Dark Disciple. The location was later mentioned in the Star Wars: Build the Millennium Falcon 70 Magazine.