In Game Description
Key to the cell of the Duke's Archive Tower.
The Archive Tower, once a trove of precious tomes and letters, became a prison after the onset of Seath's madness. The serpent men who guard the prison know not the value of what they hide. In the basement of the tower are the writhing "mistakes" of the terrifying experiments which were conducted there.
Availability
Dropped by the Serpent Soldier outside your cell
Opens
Escape from the prison after battling against Seath The Scaleless in The Duke's Archives