St. Thyrmyn Prison was an Inquisition prison in the city of Zion. As of the Year of God 898, the senior prelate in charge of the prison was Bishop Inquisitor Bahltahzyr Vekko.

St. Thyrmyn's was one of the more secure Inquisition holdings, as none of the other orders were permitted access. In addition to holding special prisoners, it was used to train inquisitors.

In February of the Year of God 898, Zhorzhet Styvynsyn, a member of the "Fist of God" resistance organization, was taken alive by the Inquisition and sent to St. Thyrmyn's. The prison was too close to the Temple of God for Merlin to stage a jailbreak. However, Owl was able to steer several nanite-loaded remotes into the prison. When activated, they killed every living thing inside in a matter of minutes.

Later that same year, St. Thyrmyn's was besieged and burned down by the revolutionaries who overthrew Grand Inquisitor Zhaspahr Clyntahn. (AST)