The Underground Prison of Morganville is where thay keep all of the vampires who are in the last stages of the Vampire Disease. The vampires there are insane, and have lost sight and most other senses. Some of them are beyond help and will remain like that forever or until their death.
Myrnin has a cell of his own, and he resides there on occasion. He is the only vampire in the prison with any bit of his sense left and the only one who is ever allowed to leave.